use persistent connections among peers in libshardcache

the behaviour can be disabled by calling 'shardcache_use_persistent_connections(cache, 0)'

the same interface has been adopted for the 'evict_on_delete' option which is not anymore
allowed to be passed in 'shardcache_create()' but needs to be overridden at runtime if necessary
use an output callback instead of a timeout (it's much faster)

and let iomux_run() take more time to capture updates otherwise it's going to be too intensive
if we only have silent filedesciptors
Merge branch 'persistent_connections'