BUG/MINOR: stats: Make stat_l variable used to dump a stat line thread local

BUG/MINOR: stats: Make stat_l variable used to dump a stat line thread local

Since ee63d4bd6 ("MEDIUM: stats: integrate static proxies stats in new
stats"), all dumped stats for a given domain, the default ones and the
modules ones, are merged in a signle array to dump them in a generic way.
For this purpose, the stat_l global variable is allocated at startup to
store a line of stats before the dump, i.e. all stats of an entity
(frontend, backend, listener, server or dns nameserver). But this variable
is not thread safe.  If stats are retrieved concurrently by several clients
on different threads, the same variable is used. This leads to corrupted
stats output.

To fix the bug, the stat_l variable is now thread local.

This patch should probably solve issues #972 and #992. It must be backported
to 2.3.