Shared buffer pool

Change buffer pool to use shared memory instead of malloc.
It allows to keep buffer pool in memory beetween mysql restarts.

We may use new variable
innodb_bp_use_shm=0|1 ( default 0)

When InnoDB starts it tries to reconnect and reuse existing buffer_pool.
When InnoDB stops it does not remove shared memory segment.

Other considerations:
1) The shared memory segment should be only reused in case of clean shutdown. It has to be reset if it was not
2) Handle changing size of buffer pool. Again if it is changed shared memory segment will need to be reinicialized
3) Make sure naming of segment allows running multiple instances on the same port. For example use port in the name of the segment
4) Have some form of version information stored. If we know new version is not compatible with stored BP we can skip reusing it.
5) Have option to reset buffer pool on start (for troubleshooting)