raft-wal

Fixes two issues:

• #33 Fixes an error on startup when a node had it's tail truncated (i.e. raft had to overwrite an uncommitted entry after a new leader took over.
• #32 Improves error handling if the disk or filesystem return errors on write or fsync.

Adds a debug dumping interface for debugging contents of segment files. See #26 and #25.

Add a fix to work with the Raft library even though we require monotonically increasing indexes but the Raft library previously relied on "gaps" being left on snapshot restore.

Also fix how we handle the first log not being index 1 since it worked at WAL level but was not properly supported by the segment package.

See #24

Fix for the verifier package to work around the fact that in practice the initial configuration log entry might not be byte-for-byte identical between peers even when everything else is correct.

Fix a major bug in GetLog where concurrent callers could return corrupted log entries due to mis-handled buffer pooling.

Refactor metrics for cleaner integration with other products.

Changes to Verification Reports so we can see when they didn't have enough data vs actually failed.

Initial release of the new version of WAL.

Note that this is still in EXPERIMENTAL state and will be being tested and verified further within our products before we recommend it for production data.