Init TBB symbols lazily but only once

[vinser52]

Description

Today UMF loads TBB library via dlopen every time the scalable pool instance is created. It is done by tbb_pool_initialize function by calling init_tbb_callbacks. We can optimize it by loading TBB library and looking for appropriate TBB symbols only once (at the first access).

API Changes

N/A

Implementation details

A scalable pool instance should not have a copy of TBB symbols, it should be a global structure of tbb symbols that is initialized lazily only once.