Move stream class to its own source/header file

The stream class will eventually be a state machine that will
manage all mutations sent over a particular stream. Since there
will be a decent amount of code for a stream we should add a
source and header file for the Stream class.

Change-Id: I9d865fd8f0129dd216bb843c23128289b79d3f99
Reviewed-on: http://review.couchbase.org/31458
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: David Liao <david.liao@couchbase.com>