[python][c++] C++ reindexer must use a single thread pool shared across all indexer objects

[beroy]

Is your feature request related to a problem? Please describe.
Currently the C++ Reindexer creates one threadpool per reindxer objects which can lead to further explosion of threads.

Describe the solution you'd like
An alternative solution is to use a single threadpool for all the reindexer objects.
To do this, the following need to done:

• Allocate a single process level threadpool
• Placing the indexer object pointer in the threadpool lookup lambda
• Few small fixes

The API changes:

• Thread pool should be part of the soma context, which already has a lifecycle.
• Config and setup can be part of the soma context setup/init. For example, it can be soma.indexer.concurrency

[beroy]

CC @bkmartinjr @johnkerl

[bkmartinjr]

Fully agree with this. A common case is two indexers used at the same time (obs & var), and it would be much nicer to have a single, shared thread pool for the entire SOMATileDBContext so that we can amortize the cost, have fewer threads, etc.

CC: @thetorpedodog

[beroy]

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