write-commands: major cleanup of write command abstraction.
write-commands: start using write commands from collection and bulk.

Lots of fallout here, time to start fixing all that up.
write-commands: add write-result to simplify result merging.
write-commands: wip.
write-commands: more command churn.
bulk: ensure hint is initialized.
write-command: check for empty insert.
write-command: set n_docuemnts on init.
collection: fix typo.
write-commands: get things working on 2.4.9.
write-command: get write commands working on 2.6.0-rc2.
write-commands: dont leak gle from collection.
tests: allow aggregate failure on mongod <2.2.