mpool.c: Track a little extra data to make ib_mpool_clear() *much* faster.
mpool.c: Track a little extra data to make ib_mpool_clear() *much* faster.

In the absence of large allocations, clear is now O(1) instead of O(n) (n = number of pages used).  This was achived by tracking the end as well as the beginning of various lists and changing clear to just concatenate lists.  I also added tracking the end of the children list.  This is currently unused but will be soon to to support ib_mpool_release().

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