don't fsync after a rewrite if appendfsync is set to no. use aof_fsycn instead of fsync where appropriate

added new option no-appendfsync-on-rewrite to avoid blocking on fsync() in the main thread while a background process is doing big I/O

minor fix for the skiplist code, resulting in a false positive with valgrind, and in general into a useless small allocation

tests suite initial support for valgrind, fixed the old test suite until the new one is able to target a specific host/port