removed the logic which took care of emptying the read buffer
 before giving a filedescriptor back to the caller

it wasn't reliable anyway since data might be still in transit and not available
and adding more overhead putting the filedescriptors in quarantine before reusing
is worthless.

In the end a caller must ensure putting back filedescriptors into the cache only
when really finished with them ... or anyway be resilient to receiving data that
the previous user didn't read (on purpose).
stop waiting for data if the shardcache instance is quitting

and also flush data (if any) in the readbuffer of filedescriptors used by the evictor thread
since the connections_pool doesn't do that for us anymore