Increase UIDs to 96 bits w/ C/F prefix - BIT-1016

- The bit-length is adjustable via redef'ing bits_per_uid.

- Prefix 'C' is used for connection UIDS (including IP tunnels) and
  'F' for files.

scripts/base/init-bare.bro

@@ -3080,6 +3080,10 @@ const snaplen = 8192 &redef;
 ## instances. If left unset, Bro will use a temporary local seed.
 const global_hash_seed: string = "" &redef;
 
+## Number of bits in UIDs that are generated to identify connections and
+## files.  The larger the value, the more confidence in UID uniqueness.
+const bits_per_uid: count = 96 &redef;
+
 # Load BiFs defined by plugins.
 @load base/bif/plugins