CDRIVER-903: Hinted cursors should still apply read prefs

This fixes an issue with the slaveOk bit not being set when using hint to query
a secondary directly.

We introduce a new is_write_command property on the cursor (private API change)
to avoid setting the slaveOk bit on write commands.

Conflicts:
	src/mongoc/mongoc-collection.c
	src/mongoc/mongoc-cursor-private.h
	src/mongoc/mongoc-cursor.c