MB-9467 Don't process documents with too large bodies

Too large documents slow down indexing and make the JavaScript engine
use too much cpu time (JSON parsing/validation), causing the whole
system to be inneficient during rebalance and failover operations or
not to be able to accomplish such operations at all.

Therefore limit the maximum size of documents passed to the JavaScript
engine, configurable via the internal parameter "indexer_max_doc_size".

Change-Id: I8e130d4c419c63f495452dab10f3e5ea96dd0db1
Reviewed-on: http://review.couchbase.org/30053
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>