MEDIUM: sink: "max-reuse" support for sink servers

MEDIUM: sink: "max-reuse" support for sink servers

Thanks to the previous commit, it is now possible to know how many events
were processed for a given sft/server sink pair. As mentioned in commit
c454296 ("OPTIM: sink: balance applets accross threads"), let's provide
the ability to restart a server connection when a certain amount of events
were processed to help better balance the load over multiple threads.

For this, we make use the of "max-reuse" server keyword which was only
relevant under "http" context so far. Under sink context, "max-reuse"
corresponds to the number of times the tcp connection can be reused
for sending messages, which in fact means that "max-reuse + 1" is the
number of events (ie: messages) that are allowed to be sent using the
same tcp server connection: when this threshold is met, the connection
will be destroyed and a new one will be created on a random thread.
The value is not strict: it is the minimum value above which the
connection may be destroyed since the value is checked after
ring_dispatch_messages() which may process multiple messages at once.

By default, no limit is enforced (the connection will be reused for as
long as it is available).

The documentation was updated accordingly.