introducing the OFX command to partially get data starting from an offset

and started implementing an asynchronous interface among peers when
fetching unowned keys from each other
updated libiomux for iomux_clear() availability
shardcache_get_async() now works

shardcache_get() actually uses shardcache_get_async() under the curtains
but it doesn't return until the value has been fully retrieved.

NOTE: this is a first working implementations, it may contain bugs,
      especially in the fault paths