TC: allow configurable poll rate

[rerpha]

as a user i would like to be able to set a configurable poll rate (sum_read_period) when talking over ADS. INTER needed this for some safety stuff.

Acceptance criteria

• Rather than being hardcoded at 200ms TC is modified to allow you to change the poll rate, but is 200ms by default

[rerpha]

#8674
ISISComputingGroup/EPICS-ioc#913

[rerpha]

needed as part of #8675