make initial replication policy configurable (most_recent, all, fail)

Config:

```
- type: push
  ...
  conflict_resolution:
    initial_replication: most_recent | all | fali
```

The ``initial_replication`` option determines which snapshots zrepl
replicates if the filesystem has not been replicated before.
If ``most_recent`` (the default), the initial replication will only
transfer the most recent snapshot, while ignoring previous snapshots.
If all snapshots should be replicated, specify ``all``.
Use ``fail`` to make replication of the filesystem fail in case
there is no corresponding fileystem on the receiver.

Code-Level Changes, apart from the obvious:
- Rework IncrementalPath()'s return signature.
  Now returns an error for initial replications as well.
- Rename & rework it's consumer, resolveConflict().

Co-authored-by: Graham Christensen <graham@grahamc.com>

Fixes https://github.com/zrepl/zrepl/issues/550
Fixes https://github.com/zrepl/zrepl/issues/187
Closes https://github.com/zrepl/zrepl/pull/592

docs/configuration.rst

@@ -13,6 +13,7 @@ Configuration
     configuration/filter_syntax
     configuration/sendrecvoptions
     configuration/replication
+    configuration/conflict_resolution
     configuration/snapshotting
     configuration/prune
     configuration/logging