SERVER-13518 generate _ids in write batches if not set in mongos
(cherry picked from commit 4b589d8aab1675e03dc7efafc32239bda660fe96)
SERVER-13518 break child batches up by size or number of documents
(cherry picked from commit 0b1994a25c85324ea413a95ace2470be3efb7db5)

Also includes SERVER-13518 make sure all tested updates are valid with update exprs
(cherry picked from commit 836b1d82810d4b7d98eed8c789f71ed45f473b85)
Merge remote-tracking branch 'upstream/v2.6' into v2.6