Add solvers statistics

[montyly]

We should have better statistics about the solver queries (timeout, ...)

dev-evm-experiments has some stats:

manticore/manticore/core/smtlib/solver.py

Lines 336 to 342 in f0d59ce

	SOLVERS_RESULTS = Manager().dict({
	'yices': 0,
	'boolector': 0,
	'cvc4': 0,
	'unknown': 0,
	'timeout': 0,
	})

But this is enabled only for the solver parallelism, and is hardcoded right now. We need to define stats, and implement a proper logging system.

The stats will help us to evaluate the solvers performance, and to identify areas of improvements

Related

• Add solver parallelism support #2404
• Provide trace of timeout/unknown  #2401