Concurrent GridStore file append operations result in file corruption / data loss, uncaught throw

Interleaving of concurrent appended writes to a GridStore file result in an uncaught throw in the driver resulting from file corruption and/or data loss due to duplicate chunk numbering when the writes cause one or more chunks to be created or added to.

Attached script synthetically reproduces the issue in a single process thread. The same issue may occur with multiple processes accessing the same GridFS collection simultaneously.