Partial revert of 0a97e68c48 (Special handling of signals for cf-key)

In that commit, the signal handler for SIGPIPE was removed. SIGPIPE
can easily happen when you pipe your input into for example grep, so
we need to restore this. We ignore the signals so any output after the
signal will just go to the void.
examples/container_key_iteration.cf: add a container key iteration example
Be silent by default if peer sends few bytes. Redmine #5919.

So that cf-serverd does not complain when portscanned,
e.g. selectservers() policy function.
Merge pull request #1743 from jimis/master

Be silent by default if peer sends few bytes. Redmine #5919.
Lower the time spent scanning lastseen DB.

The total DB access time will thus not depend on the time spent running the callback (it seems to be huge in the case of reverse DNS resolution).

This commit makes it easier to later convert the scan into an RDONLY operation.
Merge pull request #1744 from jeffali/fix/scankeys

Lower the time spent scanning lastseen DB.