thread-safe

I'm having a very hard time (as a Rust beginner) figuring out how to hack in an SASL connection with url-bot-rs, and I haven't found a dependent crate that actually uses SASL, to use as an example.

WeeChat lets you specify the SASL mechanism. A similar config option could be used here to state whether to not us

I'm opening this issue for new comers if they want to contribute.

Some tests are failing on my setup:

=== RUN   TestExpireMap_StoppedAndClose                     
--- PASS: TestExpireMap_StoppedAndClose (0.01s)
=== RUN   TestExpireMap_Curtime
--- PASS: TestExpireMap_Curtime (5.00s)
    expire_map_test.go:429: Curtime() - average diff 481423.750, max diff 1945113 (in nanosecs)
=== RUN   TestExpireMap_Curtime_WithHeavyLoad
--- FAIL: TestExpireMap_Cur