[MEDIUM] config: replace 'tcp-request <action>' with "tcp-request connection"

[MEDIUM] config: replace 'tcp-request <action>' with "tcp-request connection"

It began to be problematic to have "tcp-request" followed by an
immediate action, as sometimes it was a keyword indicating a hook
or setting ("content" or "inspect-delay") and sometimes it was an
action.

Now the prefix for connection-level tcp-requests is "tcp-request connection"
and the ones processing contents remain "tcp-request contents".

This has allowed a nice simplification of the config parser and to
clean up the doc a bit. Also now it's a bit more clear why tcp-request
connection are not allowed in backends.