Implement X509 certificate log caching

By default, each certificate is now output only once per hour. This also
should work in cluster mode, where we use the net broker-table-syncing
feature to distribute the information about already seen certificates
across the entire cluster.

Log caching is also pretty configureable and can be changed using a
range of confiuration options and hooks.

Note that this is currently completely separate from X509 events
caching, which prevents duplicate parsing of X509 certificates.

testing/btest/scripts/base/protocols/ftp/gridftp.test

@@ -10,6 +10,8 @@ module GridFTP;
 
 redef size_threshold = 2;
 
+redef X509::relog_known_certificates_after = 0secs;
+
 redef enum Notice::Type += {
     Data_Channel
 };