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.
cf-key -s has its special sig handler but the other operations need one too
Merge pull request #1745 from jeffali/cf_key/generic_sig

cf-key -s has its special sig handler but the other operations need one too