C#/Unity SDK - Add expiration for stored request durations

[bfops]

Description of Changes

Migrating clockworklabs/com.clockworklabs.spacetimedbsdk#193 since we are merging that repo into this one.

This adds configurable expiration time for request durations. This is to prevent memory leak in long-running applications where otherwise the queue would only keep growing indefinitely.

API and ABI breaking changes

None

Expected complexity level and risk

Testing

[bfops]

@jdetter or @rekhoff are you familiar with the original motivation here?

[rekhoff]

@jdetter or @rekhoff are you familiar with the original motivation here?

I am not familiar with the original motivation, no.