Added aof_backgronud_fsync() function, and use it in the bacground rewrite done handler when the fsync policy is everysec.

fixed a problem in bioOlderJobType() when there are no jobs of the specified type. Now the function returns 0 when this happens.

Background I/O library enhanced so that the main thread can query for the number of pending jobs of the specified type.

Added note about Tcl

Optimize LRANGE to scan the list starting from the head or the tail in order to traverse the minimal number of elements. Thanks to Didier Spezia for noticing the problem and providing a patch.

use bio.c instead of libeio for closing the AOF file in background. Some comment added and other minor changes.

Remove the write handler only if there are no longer objects in the output queue AND if the static buffer is empty. This bug was the cause of a possible server-stop-responding-to-client bug under some specific work load. Thanks to Pieter Noordhuis for spotting and fixing it.

Auto rewrite integer overflow fixed. Thanks to Gaurav for finding it (see issue 602 on google code), and Pieter Noordhuis for tracing it.