[#307] add package trace, integrate it with logging, and adopt it throughout zrepl

package trace:

- introduce the concept of tasks and spans, tracked as linked list within ctx
    - see package-level docs for an overview of the concepts
    - **main feature 1**: unique stack of task and span IDs
        - makes it easy to follow a series of log entries in concurrent code
    - **main feature 2**: ability to produce a chrome://tracing-compatible trace file
        - either via an env variable or a `zrepl pprof` subcommand
        - this is not a CPU profile, we already have go pprof for that
        - but it is very useful to visually inspect where the
          replication / snapshotter / pruner spends its time
          ( fixes #307 )

usage in package daemon/logging:

- goal: every log entry should have a trace field with the ID stack from package trace

- make `logging.GetLogger(ctx, Subsys)` the authoritative `logger.Logger` factory function
    - the context carries a linked list of injected fields which
      `logging.GetLogger` adds to the logger it returns
    - `logging.GetLogger` also uses package `trace` to get the
      task-and-span-stack and injects it into the returned logger's fields

rpc/grpcclientidentity/authlistener_grpc_adaptor.go

@@ -99,10 +99,23 @@ func (*transportCredentials) OverrideServerName(string) error {
 	panic("not implemented")
 }
 
-type ContextInterceptor = func(ctx context.Context) context.Context
+type ContextInterceptorData interface {
+	FullMethod() string
+	ClientIdentity() string
+}
 
-func NewInterceptors(logger Logger, clientIdentityKey interface{}, ctxInterceptor ContextInterceptor) (unary grpc.UnaryServerInterceptor, stream grpc.StreamServerInterceptor) {
-	unary = func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (resp interface{}, err error) {
+type contextInterceptorData struct {
+	fullMethod     string
+	clientIdentity string
+}
+
+func (d contextInterceptorData) FullMethod() string     { return d.fullMethod }
+func (d contextInterceptorData) ClientIdentity() string { return d.clientIdentity }
+
+type Interceptor = func(ctx context.Context, data ContextInterceptorData, handler func(ctx context.Context))
+
+func NewInterceptors(logger Logger, clientIdentityKey interface{}, interceptor Interceptor) (unary grpc.UnaryServerInterceptor, stream grpc.StreamServerInterceptor) {
+	unary = func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
 		logger.WithField("fullMethod", info.FullMethod).Debug("request")
 		p, ok := peer.FromContext(ctx)
 		if !ok {
@@ -115,10 +128,18 @@ func NewInterceptors(logger Logger, clientIdentityKey interface{}, ctxIntercepto
 		}
 		logger.WithField("peer_client_identity", a.clientIdentity).Debug("peer client identity")
 		ctx = context.WithValue(ctx, clientIdentityKey, a.clientIdentity)
-		if ctxInterceptor != nil {
-			ctx = ctxInterceptor(ctx)
+		data := contextInterceptorData{
+			fullMethod:     info.FullMethod,
+			clientIdentity: a.clientIdentity,
 		}
-		return handler(ctx, req)
+		var (
+			resp interface{}
+			err  error
+		)
+		interceptor(ctx, data, func(ctx context.Context) {
+			resp, err = handler(ctx, req) // no-shadow
+		})
+		return resp, err
 	}
 	stream = func(srv interface{}, ss grpc.ServerStream, info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
 		panic("unimplemented")

rpc/grpcclientidentity/grpchelper/authlistener_grpc_adaptor_wrapper.go

@@ -50,7 +50,7 @@ func ClientConn(cn transport.Connecter, log Logger) *grpc.ClientConn {
 }
 
 // NewServer is a convenience interface around the TransportCredentials and Interceptors interface.
-func NewServer(authListener transport.AuthenticatedListener, clientIdentityKey interface{}, logger grpcclientidentity.Logger, ctxInterceptor grpcclientidentity.ContextInterceptor) (srv *grpc.Server, serve func() error) {
+func NewServer(authListener transport.AuthenticatedListener, clientIdentityKey interface{}, logger grpcclientidentity.Logger, ctxInterceptor grpcclientidentity.Interceptor) (srv *grpc.Server, serve func() error) {
 	ka := grpc.KeepaliveParams(keepalive.ServerParameters{
 		Time:    StartKeepalivesAfterInactivityDuration,
 		Timeout: KeepalivePeerTimeout,