fixed memory leak in memory_cache
give more opportunities for work stealing
downgraded double KILL_PROXY msg warning to info

this actually has no effect on client code, but is merely a minor
Boost.Actor issue, so it souldn't be a warning in the first place
handle invalid_actor in sync send + anon_send_exit

this patch checks in anon_send_exit for invalid_actor and
turns it into a NOP if needed, sync_send throws
std::invalid_argument to have at least a defined behavior
fixed performance issue with prioritizing policy

this patch fixes an issue with the prioritizing policy where the
low-priority cache could slow down the performance of dequeue
operations to the point where the system seems to stop;
this patch also removes the implicit addition of the priority_aware
flag whenever the detached flag is used