Fix for znc-clientbuffer#13

[glechapelain]

o Do not update timestamp upon sending messages to the client
o Always update timestamp regardless of the type of message we get back from the client

[glechapelain]

Back to the drawing board:

I am totally getting messages played back that I would not expect (namely: mines! 😅)
I'll take a deeper look,

[glechapelain]

Back to the drawing board:

I am totally getting messages played back that I would not expect (namely: mines! 😅) I'll take a deeper look,

actually it's not exactly `mine' in that it was from a different user on a different ZNC session.
Nevertheless they date back over 1/2 an hour before my session went offline.

I was expecting the pong to be every few minutes at most...