This patch collects all previous patches into one.
The main problem was due to that there is are two variables -
dflt_key_cache and sql_key_cache with have more or less duplicate
function. The reson for the bug was that the default value in the key
cache hash was set to dflt_key_cache, then sql_key_cache was set to a
new key cache object, and then dflt_key_cache was set to sql_key_cache
which was different from the dflt_key_cache_var. After sending SIGHUP,
the server was using the original default value for the key cache hash,
which was different from the actual key cache object used for the
default key cache.
Introduced a new free blocks list. Free blocks are now re-used before
new blocks are allocated from the pool. There is a new status variable
which can be queried by "show status like key_blocks_unused".
