- add parameter to `conn_is_alive()` cfilter method that returns
  if there is input data waiting on the connection
- refrain from re-using connnection from the cache that have
  input pending
- adapt http/2 and http/3 alive checks to digest pending input
  to check the connection state
- remove check_cxn method from openssl as that was just doing
  what the socket filter now does.
- add tests for connection reuse with special server configs

- msh3: fix parameter type wrong in is_alive check
- nss, securetransport: remove own `check_cxn` method, as
  other TLS backends also no longer do their own.