fixed memory leak introduced with the previous commit. Many thanks to Pieter Noordhuis for spotting it in no time

Fixed issue #503. MONITOR + QUIT could crash the server, there are actually other interactions that could have the same effect (for instance Pub/Sub).

Fixed issue #435 and at the same time introduced explicit ping in the master-slave channel that will detect a blocked master or a broken even if apparently connected TCP link.

implemented a different approach to IO scheduling, so object->storage is no longer used, instead there is a queue and hash table of IO tasks to process, and it is always possible to know what are the scheduled and acrtive IO operations against every single key.

LRANGE converted into a COW friendly command. Some refactoring, comment, and new addReply*() family function added in the process.

first attempt to non blocking implementation of slave replication and SYNC bulk data download. Never compiled so far...

modified a bit addReply() to play better with copy on write now that we have a static buffer. Changed the name of a function from _ensureFileEvent() to _installWriteEvent().

When creating the readable event results in an error (this happens when
the server hits OS limits), the client was not added to the list of
clients when freeClient was called. This results in an assertion error.
It is better to check this condition first and free the client
immediately when this condition occurs. Port of 00a90feb.

slave with attached slaves now close the conection to all the slaves when the connection to the master is lost. Now a slave without a connected link to the master will refuse SYNC from other slaves. Enhanced the replication error reporting. All this will fix Issue 156