* Fix handling BSON serializer differences between pymongo's `bson` and standalone `bson` codec.
* Handle `CorruptGridFile` error in GridFS backend
* Fix cache path expansion for user directories (`~/...`) for SQLite and filesystem backends
* Fix request normalization for request body with a list as a JSON root
* Skip normalizing a JSON request body if it's excessively large (>10MB) due to performance impact
* Fix some thread safety issues:
* Fix race condition in SQLite backend with dropping and recreating tables in multiple threads
* Fix race condition in filesystem backend when one thread deletes a file after it's opened but
before it is read by a different thread
* Fix multiple race conditions in GridFS backend