fix: Make flush thread a daemon thread to prevent shutdown hang

[shawkinsmezmo]

Convert the flush thread from a threading.Timer to a regular threading.Thread with daemon=True. Daemon threads don't prevent the application from gracefully shutting down.

A thread can be flagged as a “daemon thread”. The significance of this flag is that the entire Python program exits when only daemon threads are left. The initial value is inherited from the creating thread. The flag can be set through the daemon property or the daemon constructor argument.

[dkhokhlov]

the main change looks ok.

I just noticed this runtime error handling may block for long time doing request retries even with should_block=False:
https://github.com/logdna/python/pull/102/files#diff-d98932f491dbf58581e648d2178063e919d9c1c80b58afa5e47bdc88c25211caR137

harmless?

[shawkinsmezmo]

the main change looks ok.

I just noticed this runtime error handling may block for long time doing request retries even with should_block=False: https://github.com/logdna/python/pull/102/files#diff-d98932f491dbf58581e648d2178063e919d9c1c80b58afa5e47bdc88c25211caR137

harmless?

Should be harmless, yes. The only situation in which a RuntimeException would be thrown when attempting to submit the flush request to the thread pool would be if the pool was already shutdown, which can only happen during close, in which case there is already a final flush request that is queued up as part of the close workflow. Beyond that, the thread pool is only shut down after it is guaranteed that there is no more data actively being buffered, and all flush requests that are pending / created have completed.

[dkhokhlov]

lgtm