No chance to run a reactor GC if memcached GC runs longer than `gc_interval`

[ymmt2005]

yrmcds has two garbage collection mechanisms:

1. Collecting invalidated socket structs (reactor GC)
2. Collecting cached objects (memcached GC)

Since the latter prevents the former from running, if gc_interval is shorter than the time for the memcached GC, yrmcds does not give a chance to run the reactor GC, which leads to unfreed memory.

cf. https://github.com/cybozu/yrmcds/issues/7#issuecomment-26306917