BUG/MINOR: call EXTRA_COUNTERS_FREE() before srv_free_params() in srv_drop()

BUG/MINOR: call EXTRA_COUNTERS_FREE() before srv_free_params() in srv_drop()

As seen with the last changes to counters allocation, the move of the
counters storage to the thread group as operated in commit 04a9f86a85
("MEDIUM: counters: add a dedicated storage for extra_counters in various
structs") causes some random errors when using ASAN, because the extra
counters are freed in srv_drop() after calling srv_free_params(), which
is responsible for freeing the per-thread group storage.

For the proxies however it's OK because free calls are made before the
call to deinit_proxy() which frees the per_tgrp area.

No backport is needed, this is purely 3.4-dev.