Clear rx buffer before call

[MightyPork]

This helps with error recovery on unreliable physical connections.

An older version of this workaround was running for over 2 years now in several industrial installations

Relates to #60

[MightyPork]

haha we now have three independent PRs trying to fix that 2020 issue.

• fix: clear cache before call #196 seems like an extremely complicated way to do the same and frankly I don't understand the code
• Since modbus RTU is a request and response protocol, make sure the read buffer is always clear before sending another request #189 does the right thing, but misses the TCP service

[ColinFinck]

@uklotzde Looks good to me! Can you approve the GitHub Actions workflows, so that we can have a last check before this gets merged?

This should indeed supersede #189 and #196

[uklotzde]

@ColinFinck Thanks for stepping in again!

Please proceed as proposed ;)