Periodically print Z3 statistics

[konnov]

Closes #2991. This PR adds a debugging feature that helps me to get some hints about the SMT slowdowns:

• Print Z3 statistics when a z3 context is disposed, e.g., on exit.
• Print Z3 statistics every 60 seconds.

The statistics is only printed when --debug is on. Currently, the timeout period is ad-hoc. Once #2990 is merged, we can introduce better flags in the tuning options.

• Tests added for any new code -- not really, I don't know how to test this effectively.
• Ran make fmt-fix (or had formatting run automatically on all files edited)
• Documentation added for any new functionality -- no need, it's basically integrated into --debug
• Entries added to ./unreleased/ for any new functionality

[thpani]

lgtm 👍