MEDIUM: stick-tables: flush old entries upon soft-stop

When a process with large stick tables is replaced by a new one and remains
present until the last connection finishes, it keeps these data in memory
for nothing since they will never be used anymore by incoming connections,
except during syncing with the new process. This is especially problematic
when dealing with long session protocols such as WebSocket as it becomes
possible to stack many processes and eat a lot of memory.

So the idea here is to know if a table still needs to be synced or not,
and to purge all unused entries once the sync is complete. This means that
after a few hundred milliseconds when everything has been synchronized with
the new process, only a few entries will remain allocated (only the ones
held by sessions during the restart) and all the remaining memory will be
freed.

Note that we carefully do that only after the grace period is expired so as
not to impact a possible proxy that needs to accept a few more connections
before leaving.

Doing this required to add a sync counter to the stick tables, to know how
many peer sync sessions are still in progress in order not to flush the entries
until all synchronizations are completed.
MINOR: tcp: add new "close" action for tcp-response

This new action immediately closes the connection with the server
when the condition is met. The first such rule executed ends the
rules evaluation. The main purpose of this action is to force a
connection to be finished between a client and a server after an
exchange when the application protocol expects some long time outs
to elapse first. The goal is to eliminate idle connections which
take signifiant resources on servers with certain protocols.
MINOR: payload: provide the "res.len" fetch method

This fetch method returns the response buffer len, similarly
to req.len for the request. Previously it was only possible
to rely on "res.payload(0,size) -m found" to find if at least
that amount of data was available, which was a bit tricky.