enforce encapsulation by breaking up replication package into packages

not perfect yet, public shouldn't be required to use 'common' package to
use replication package

cmd/replication/common/common.go

@@ -0,0 +1,91 @@
+package common
+
+import (
+	"context"
+	"io"
+
+	"github.com/zrepl/zrepl/cmd/replication/pdu"
+	"github.com/zrepl/zrepl/logger"
+)
+
+type contextKey int
+
+const (
+	contextKeyLog contextKey = iota
+)
+
+type Logger = logger.Logger
+
+func ContextWithLogger(ctx context.Context, l Logger) context.Context {
+	return context.WithValue(ctx, contextKeyLog, l)
+}
+
+func GetLogger(ctx context.Context) Logger {
+	l, ok := ctx.Value(contextKeyLog).(Logger)
+	if !ok {
+		l = logger.NewNullLogger()
+	}
+	return l
+}
+
+type ReplicationEndpoint interface {
+	// Does not include placeholder filesystems
+	ListFilesystems(ctx context.Context) ([]*pdu.Filesystem, error)
+	ListFilesystemVersions(ctx context.Context, fs string) ([]*pdu.FilesystemVersion, error) // fix depS
+	Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, io.ReadCloser, error)
+	Receive(ctx context.Context, r *pdu.ReceiveReq, sendStream io.ReadCloser) error
+}
+
+type FilteredError struct{ fs string }
+
+func NewFilteredError(fs string) *FilteredError {
+	return &FilteredError{fs}
+}
+
+func (f FilteredError) Error() string { return "endpoint does not allow access to filesystem " + f.fs }
+
+type ReplicationMode int
+
+const (
+	ReplicationModePull ReplicationMode = iota
+	ReplicationModePush
+)
+
+type EndpointPair struct {
+	a, b ReplicationEndpoint
+	m    ReplicationMode
+}
+
+func NewEndpointPairPull(sender, receiver ReplicationEndpoint) EndpointPair {
+	return EndpointPair{sender, receiver, ReplicationModePull}
+}
+
+func NewEndpointPairPush(sender, receiver ReplicationEndpoint) EndpointPair {
+	return EndpointPair{receiver, sender, ReplicationModePush}
+}
+
+func (p EndpointPair) Sender() ReplicationEndpoint {
+	switch p.m {
+	case ReplicationModePull:
+		return p.a
+	case ReplicationModePush:
+		return p.b
+	}
+	panic("should not be reached")
+	return nil
+}
+
+func (p EndpointPair) Receiver() ReplicationEndpoint {
+	switch p.m {
+	case ReplicationModePull:
+		return p.b
+	case ReplicationModePush:
+		return p.a
+	}
+	panic("should not be reached")
+	return nil
+}
+
+func (p EndpointPair) Mode() ReplicationMode {
+	return p.m
+}