Configurable Writer Threads for Slice Writer

The writer threads for a slice writer are configurable now.
Each thread will have its own connection to the database.

Change-Id: Idb6571fea89328d1460eee8f34ead02f7e6a7e8a
Reviewed-on: http://review.couchbase.org/39440
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>