Bug 2890: Chunk AppendEntries when single payload size exceeds threshold

Bug 2890: Chunk AppendEntries when single payload size exceeds threshold

Utilizes the MessageSlicer in AbstractLeader to slice/chunk AppendEntries
messages whose single log entry payloas exceeds the max size threshold.
The MessageAssembler is used in the Follower to re-assemble.

For efficiency, with multiple followers, the AbstractLeader reuses the
FileBackedOutputStream containing the serialized AppendEntries data.
However, since the MessageSlicer takes ownership of the FileBackedOutputStream
and cleans it up when slicing is complete, I added a
SharedFileBackedOutputStream class that maintains a usage count and
performs cleanup when the usage count reaches 0. The AbstractLeader maintains
a Map of SharedFileBackedOutputStream instances keyed by log index.

The FollowerLogInformation keeps track of whether or not slicing is in
progress for the follower. Same as with install snapshot, we only want to send
empty AppendEntries as heartbeats.

Change-Id: Id163944b9989f6cb39a6aaaa98d1f3c4b0026bbe
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>