Handle OP_MSG requests exceeding MaxMessageSizeBytes

[domodwyer]

NOTE: this issue is for an experimental feature, and does not affect the production driver.

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The OP_MSG specification states:

Each OP_MSG MUST NOT exceed the maxMessageSizeBytes as configured by the MongoDB Handshake.

Currently the experimental support for OP_MSG (contributed by @feliixx - thanks!) does not gracefully handle a message exceeding maxMessageSizeBytes - in the future this limit should be handled transparently by the driver.

In socket.go we return an error from sendMessage() when the limit is exceeded - ideally the driver should split the requests into messages that do not exceed this limit internally.