Seqlist purging - move decision of which seqno to purge upto from Seqlist

See parent MB - we have implemented a quick fix to simply have the SequenceList stop at one element before the end of the list when purging (http://review.couchbase.org/#/c/80417/). However, this isn't a clean solution - it mixes policy decisions (what to purge) with the container providing storage.

We should implement a cleaner solution which maintains the consistency of the SequenceList class as a generic, ordered container, and moves the decision of what to purge to the calling object (i.e. EphemeralVBucket).