Add backpressure to send buffer

[cbrewster]

Why

Currently the send buffer has no backpressure, so if a user of the river client sends too many messages too quickly, the send buffer will overflow and the client will error out.

Rather than relying on users to rate limit themselves, we introduce backpressure so client naturally will only send messages as fast as the river client/server is able to consume them.

What changed

• Add a condvar to the message buffer to allow waiting for buffer space to come available when the buffer is full
• Add a closed bit to the message buffer so when a session is shut down, we have a way of unblocking all the futures that are stuck waiting for space in the buffer
• Close the message buffer when the session is closed

Test plan

• Added a test which sends 2 * MAX_MESSAGE_BUFFER_SIZE messages to a river server. Previously this resulted in an error.
• Added some unit tests for the MessageBuffer to ensure backpressure works and closing works

[cbrewster]

Current dependencies on/for this PR:

• main:
  • PR Add backpressure to send buffer #97 👈

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[lhchavez]

heck yeah!