Object approximated LRU algorithm enhanced / fixed / refactored. This is used for the VM currently but will soon be used for maxmemory expiring.

maxmemory fixed, we now try to release memory just before we check for the memory limit. Before fixing there was code between the attempt to free memory and the check for memory limits, and this code could result into allocations going again after the memory limit.

This should fix Issue 332: when there is a background process saving we still allow the hash tables to grow, but only when a critical treshold is reached. Formerly we prevented the resize at all triggering pathological O(N) behavior. Also there is a fix for the statistics in INFO about the number of keys expired

Fixed segfault in freeMemoryIfNeeded due to the fact that keys are now sds strings and not objects in the main hash table, thanks to Anthony Lauzon for spotting the bug and providing a patch.

The pid file is now created only after the server is correctly initialied. It is also removed on sigterm and when the stack trace is produced after a sigbus or a sigsegv. This two changes should fix the Issue 175

This avoids the extra allocation of sizeof(double) for storing the score
of a zset entry in the hash table. Saves sizeof(double) + malloc
overhead = approx. 16 bytes per zset entry.

support for write operations against expiring keys, by master-controlled expiring in replication and AOF synthesizing DEL operations

use the function deprecated attribute if compiling with GCC to get warnings for malloc/free usages. We always want to use our zmalloc/zfree versions for memory usage tracking

don't use object sharing inside I/O threads, as a fix for a well known instability of VM introduced with the new object sharing code

networking related stuff moved into networking.c

moved more code

more work on layout of source code

SDS instantaneuos memory saving. By Pieter and Salvatore at VMware ;)

cleanly compiling again after the first split, now splitting it in more C files

moving more things around... work in progress

split replication code

splitting more

Sets split

Hash split

replication split

even more splitting

more splitting

minor change