SERVER-13681: Remove lock that was an incorrect workaround for Azure Drive write size limitations.

- Limit flushes to 2MB chunks to prevent hitting a limitation in
Azure Network Drives (XDrive) which have a 2MB file write size limitation.
SERVER-13729: Fix Data File Growth on Windows

Reuse an existing lock to use as a lock to handle the race condition
between _flushAll, and the memory mapped file being closed instead
of holding LockMongoFilesxxx for an excessively long time.