Add throttle option for incremental index updates

Currently the updater seems to proceed significantly faster than the
compactor, which has a retry phase not very efficient when there's many
small updates during the initial compaction phase, which makes the
compactor retry phase apply too many small log files to the new index,
causing fragmentation on the new index to be high by the time compaction
finishes. This in turn causes the compaction scheduler to schedule yet
another compaction shortly after a compaction finishes.

So for now make the updater sleep for a short period before applying each
batch if compaction is currently running and in the retry phase. Not ideal
and this is a temporary workaround which can ne disabled or adjusted at
any time by setting the sleep interval (couch_config setting throttle_period).

A better solution is to make the retry phase more efficient, faster and
causing much less fragmentation. Amongst other possibilities, merging log
files into a single batch and applying it all at once makes a big difference
in regards to efficiency and speed - basically restoring what we had until
revision 1969a700dd9dfe072608c7b5fcf324eac8ab008d but in a fully correct
way.

Change-Id: I374cbbc0b7fb01257457c4b3f81b72a332afb6ff
Reviewed-on: http://review.couchbase.org/33991
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>