MEDIUM: proxy: add lock for global accesses during default free

MEDIUM: proxy: add lock for global accesses during default free

This patch is similar to the previous one, but this time it deals with
functions related to defaults proxies instances. Lock PROXIES_DEL_LOCK
is used to protect accesses on global collections.

This patch will be necessary to implement dynamic backend deletion, even
if defaults won't be use as direct target of a "del backend" CLI.
However, a backend may have a reference on a default instance. When the
backend is freed, this references is released, which can in turn cause
the freeing of the default proxy instance. All of this will occur at
runtime, outside of thread isolation.