REORG: connection: move the hash-related stuff to connection.c

REORG: connection: move the hash-related stuff to connection.c

We do not really need to have them inlined, and having xxhash.h included
by connection.h results in this 4700-lines file being processed 101 times
over the whole project, which accounts for 13.5% of the total size!
Additionally, half of the functions are only needed from connection.c.
Let's move the functions there and get rid of the painful include.

The build time is now down to 6.2s just due to this.