MINOR: checks: maintain counters of active checks per thread

MINOR: checks: maintain counters of active checks per thread

Let's keep two check counters per thread:
  - one for "active" checks, i.e. checks that are no more sleeping
    and are assigned to the thread. These include sleeping and
    running checks ;

  - one for "running" checks, i.e. those which are currently
    executing on the thread.

By doing so, we'll be able to spread the health checks load a bit better
and refrain from sending too many at once per thread. The counters are
atomic since a migration increments the target thread's active counter.
These numbers are reported in "show activity", which allows to check
per thread and globally how many checks are currently pending and running
on the system.

Ideally, we should only consider checks in the process of establishing
a connection since that's really the expensive part (particularly with
OpenSSL 3.0). But the inner layers are really not suitable to doing
this. However knowing the number of active checks is already a good
enough hint.