MEDIUM: peers: Fix a peer stick-tables synchronization issue.

MEDIUM: peers: Fix a peer stick-tables synchronization issue.

During the stick-table teaching process which occurs at reloading/restart time,
expiration dates of stick-tables entries were not synchronized between peers.

This patch adds two new stick-table messages to provide such a synchronization feature.

As these new messages are not supported by older haproxy peers protocol versions,
this patch increments peers protol version, from 2.0 to 2.1, to help in detecting/supporting
such older peers protocol implementations so that new versions might still be able
to transparently communicate with a newer one.

[wt: technically speaking it would be nice to have this backported into 1.6
 as some people who reload often are affected by this design limitation, but
 it's not a totally transparent change that may make certain users feel
 reluctant to upgrade older versions. Let's let it cook in 1.7 first and
 decide later]