WaitForSocketCondition: rename, handle EINTR

- Rename WaitForSocketCondition to SingleWaitForFdCondition
  - Avoid potentially infinite loop, if we continue to get interrupted
  - Handle eintr correctly
  - Avoid the poller try/finally, as the poller object gets destroyed
    anyway
- Provide a new WaitForFdCondition
  - Using retry, guarantee to continue checking until the timeout
    expires
  - Needs an extra helper class, as it uses retry in a very custom way
    (no sleep happens, because the poller sleeps by itself)

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>