MB-12067 [5/6]: pause/resume visitor support for epStore & HashTable

Add new visiting methods to HashTable and EventuallyPersistentStore to
allow a client to pause the visit at a particular position, and then
resume from approximately the same position at a later time.

The methods operate in a similar way to the existing visit() /
visitDepth() methods, with the addition of a start_pos parameter to
specify where to start at, and returning the position the visitation
got to.

Clients would typically call pauseResumeVisit() with a default
Position initially (to start from the beginning), and then when they
want to pause they return false from their visit() method.

Change-Id: I2542efc165e3200f0cc74d8120a16e2175f1341d
Reviewed-on: http://review.couchbase.org/41498
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Dave Rigby <daver@couchbase.com>