Switch the pool of tile data to boost::singleton_pool

Using usual malloc/free causes huge memory fragmenation, in the result
Krita eats all the memory of the system until some stable fragmented
state is reached.

There seems to be a regression: the first stroke is sometimes delayed.
Needs testing.

Testcase:
Image 8k x 8k, 2 layer
Brush 1k x 1k

Paint a dozen of strokes with undo disabled. If the memory continually
grows, it means the RAM gets fragmented. This image should not take much
more than 1.5 GiB (and massif will report it)

CCMAIL:kimageshop@kde.org