Slice Writer Changes for Concurrency

ForestDB requires each thread to have its own handle or
all requests be serialized using a single thread.

Slice Writer now has a buffered channel to queue requests
to ForestDB and these are then serialized.

External Interface remains the same and there are no
changes to Flusher.

Change-Id: I2a3b4601e70500f239113abb0ec7983d141537be
Reviewed-on: http://review.couchbase.org/39304
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>