Refactoring of partitioning interface by moving partitioning specific
functions of class handler to an own class Partition_handler.

Also refactoring the generic partitioning engine ha_partition,
by moving reusable code into a new class Partition_helper so it can
be used by other partitioning aware engines, such as native innodb
partitioning (wl#6035).

Includes removing #ifdef WITH_PARTITION_STORAGE_ENGINE in server code,
so that parsing etc. always support partitioning and it is engine
support that determines if partitioning is supported or not.

Refactoring ha_partition so that it can be built as a plugin,
to show it is separated from the server code. Also moved the code
to storage/partition directory.

rb#7546
Approved by Mikael Ronström,
runtime parts approved by Dmitry Lenev,
ndb parts approved by Magnus Blåudd