[libcheckers] async path checking in the framework

[libcheckers] async path checking in the framework

The directio path checker was recently moved to aio API, but the behaviour is still synchronous : io_getevents() blocks until (long) timeout expires.

Truely asynchronous behaviour imposes to
o lower the io_getevents timeout to mininum
o treat a new "pending" checker return status in the daemon (reschedule the checker early for a new io_getevents until the "long" timeout expires)

This patch explores this. This approach has the nice effect to behave well with still-synchronous checker. The daemon should see no regression. But multipath, which assumes synchronous path checking, is for now broken ... to be repaired.

Please comment abundantly this approach, before I start moving ahead.