This PR revises smt.threads behavior to use a pool of worker threads that collaborate on a shared search tree.
The search tree allows some diversification of search, which is helpful for unstable theories such as NIA.
It also integrates inprocessing, which was not available so far. In-processing kicks in after a couple of rounds of short timeout search and is run once. While current measurements show bounded advantage of in-processing, it has a potential advantage on incremental scenarios where the smt core isn't by default set up to have strong pre-processing.