Refactor seqs cache implementation

The seqs cache is currently part of view_group state. The cache
will be later moved to its on module or inside the dcp_client module.
The cache needs to be updated everytime when any get_seqs() results
are processed inside view_group to avoid duplicate get_seqs() calls.

Refactored seqs_cache to make it separated out from process state variable.
Any function should be able to update the cache without having reference
to state variable. Hence, moved the location of seqs_cache to process table.

Change-Id: Ic3038dd7d7fd91e15c09f11d15654582f2ee23b0
Reviewed-on: http://review.couchbase.org/40407
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>