Move code into the more generic module

This is a refactoring without a functional change. Code from the specific
mapreduce views merger is moved into the more general index merger.

Change-Id: I119109624e1a0c4bd5b5f55f37f3f74b7e8cc9f6
Reviewed-on: http://review.couchbase.org/39638
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>