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.
(cherry picked from commit f4f36f2010679e9a112ca52b4a571c8a763af2f9)
examples/container_key_iteration.cf: add a container key iteration example

(cherry picked from commit c7fa1ae141d32a4827d0830812dba49c03dd03e1)
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.

(cherry picked from commit 9f4a45c6e56ab1fe8cf877742293368654aff940)
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.

(cherry picked from commit bec9cd1d2142dda7836b1eebb982e7297dafebf1)