Mongor requires knowledge of when a client session starts and ends on the shadow cluster.

This is approximately equal to the lifetime of a "query serving thread" for ServiceExecutorSyncImpl.

Spike hooks.

The format and reader will need to be extended for new event "types".

This can extend TrafficRecorderPacket, possibly with a variant field containing either a recorded message, or some other event. This could be discriminated by a preceding flag (e.g., a newly added uint8_t) which informs the reader which type to expect.

Alternatively, as the message is BSON, a simple solution may be to have special messages, with known first field names (in the same manner as commands). This is likely the simplest implementation.