Add buffer limit for stored requests

[Kyle-Verhoog]

Currently the test agent will store all requests sent to it, without limit. This causes noticeable performance issues after several thousand traces are collected since the agent works by scanning backwards through all traces when performing operations.

The default should be to only store up to a fixed number of requests with an option to configure it.

[Kyle-Verhoog]

@colin-higgins had the idea to also optionally flush them to disk