Add support for automatically asserting a serial break after every write

[FreddieAkeroyd]

Add support for asserting a serial break after every write, for devices that use it as a command terminator.

[AppVeyorBot]

:x: Build asyn 1.0.241 failed (commit https://github.com/epics-modules/asyn/commit/c7e99d0fe2 by @FreddieAkeroyd)