MEDIUM: proxy: implement purge backend

MEDIUM: proxy: implement purge backend

Add a new CLI command "purge backend", similar to "purge server". Its
objective is to remove a proxy instance at runtime, or if not possible
due to a transient error, flag the proxy for background deletion.

To implement it, a new flag PR_FL_TO_PURGE is defined. It is tested
inside proxy purge task to periodically test backend deletion via
be_check_for_deletion() until the operation can be completed.

Also, be_check_for_deletion() has been extended to add a pr_del_mode as
parameter. When using purge, backend server list can still be non empty,
at the condition that every entries in it are flagged for background
deletion as well.

A backend deletion can be cancelled by using publish/unpublish commands.
This will simply remove PR_FL_TO_PURGE and log a message to report the
cancellation.