ArenaAllocator complete (Dbspj)

and i created a new DataBuffer2, which has pool-type as template argument.
it was "impossible" to alter DataBuffer to do this...it was simply to hard-wired to ArrayPool. too bad.

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so now everything in Dbspj can/will be using the ArenaAllocator...
rationale for ArenaAllocator is really not to have a "quick" release (the code still releases each object individually...to reset magic values) but that it's a kind of variable length allocation strategy that provide great locality of data...

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also pushed infrastructure for "result-set correlation".
Dbspj will (at least for now) when joining only return rows once so in the case of a 2-way join, a SQL result set will return the rows from the first table one time *for each* match in second table.
Dbspj will not...(but ha_ndbcluster will have to do this...when presenting things for mysqld) and since it will not, there has to be a way to correlate rows from the 2 tables.
this we call "result-set correlation".