Stm32: implement async flush for UART

[qwerty19106]

1. It implement non-blocking async flush for UART by enable Transmission complete interrupt.
2. It consist fix for half-duplex: either tx or rx can be enabled one moment. The fix(stm32): disable transmitter during during half-duplex flush #3299 can be closed after merging it, because flush should get non-mutable reference. Flush should wait only, no more.

[qwerty19106]

rebased

[Dirbaio]

i've added on-hardware tests that test flush (just testing that it doesn't hang, it's hard to test that it actually waits...), let's see if they pass.

[Dirbaio]

tests pass! thanks for the PR :D