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Memcahed implementation of the memory caching API

A new module that provides a new caching method using memcached servers was added.
It provides a way to access memcached servers using the existing memcache API.

Advantages over the existing "localcache" module:

• memory costs are no longer on the server
• many servers may be used so the memory is virtually unlimited
• the cache is persistent so a restart of the server will not affect the cache
• memcached is an open-source project so it can be used to exchange data with various other applications
• servers may be grouped together (e.g. for security purposes : some can be inside a private network, some can be in a public one).

Limitations:

• keys (in key:value pairs) may not contain spaces or control characters

Requirements:

• libmemcahed is a dependency of the module ( see docs)
• memcached servers must exist somewhere accessible via the network.

Usage:

• memcached module must be loaded
• server parameters must be set

E.g:

	...
	modparam("memcached", "server","group1 = localhost:9999,127.0.0.1" );
	modparam("memcached", "server","y = random_url:8888" );
	...

This creates two groups of servers( one called "group1" and one called "y").

Calls to the two groups will be done via the memcache API by passing the first parameter as "memcached_($GROUP_NAME)".

E.g:

	
	...
	cache_store("memcached_group1","key","$ru value");
	...

This stores "key"="$ru value" on the group denoted by "group1". The key is only stored on one of the servers in a consistent fashion (subsequent calls with "key" will be stored, deleted from the same server).

	...
	cache_fetch("memcached_y","key",$avp(i:10));
	cache_remove("memcached_y","key");
	...

This attempts to fetch/remove key from the group named "y".