Remove the siphash->hmac-md5 switch after 36 bytes.

Currently, siphash is used for strings up to 36 bytes. hmac-md5 is used
for longer strings.

This switch-over is a remnant of the previous hash-function that was
used, which apparently was slower with longer input strings.

This change serves no purpose anymore. I performed a few performance tests
on strings of varying sizes:

For a 40 byte string with 10 million iterations:

siphash: 0.31 seconds
hmac-md5: 3.8 seconds

For a 1080 byte string with 10 million iterations:

siphash: 4.2 seconds
hmac-md5: 17 seconds

For a 18360 byte string with 10 million iterations:

siphash: 69 seconds
hmac-md5: 240 seconds

Hence, this commit removes the use of hmac-md5.

This change causes reordering of lines in a few logs.

This commit also changes the datastructure for the seed in probabilistic/Hasher
to get rid of a type-punning warning.

src/Hash.h

@@ -6,8 +6,6 @@
 
 #include <stdlib.h>
 
-#define UHASH_KEY_SIZE 36
-
 class BroString;
 
 typedef uint64_t hash_t;