asyn: throttle rate of serial commands to device

[FreddieAkeroyd]

As a developer I would like a way to control the maximum rate at which commands are sent to a device. Sometime a device manual states that there should be a minimum time between commands, or a maximum rate of commands. It is possible to add delays into Db files and/or stream device protocol, but this can get messy or things get missed.

Acceptance Criteria

• function asynInterposeThrottleConfig(port, address, min_delay_between_commands) available
• documented at https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/Asyn-Interpose-Functions

To Test

• pick an IOC that has a lot of 1 second scan loops
• add something like asynInterposeThrottleConfig("L0",0, 0.1) to the st.cmd after port L0 is created
• set the trace mask to 0xD to include filter output as per https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/ASYN-Trace-Masks-(Debugging-IOC,-ASYN)
• run the ioc system tests
• check logs for lines like asynInterposeThrottle:writeIt L0 delaying 0.097989 seconds and that the timestamps on reads and writes look ok i.e. no two write lines occur less than the specified delay apart

[rerpha]

Worked nicely for #6243 and clearly throttles commands