Short-circuit evaluation in validity checks

[jcoupey]

In numerous situations, deciding if a local search move is valid is the result of calling several functions in a row to check validity with regard to skills, capacity, time windows etc.

The current code-base usually uses something along the lines of:

bool valid = first_check();
valid &= another_check();
...
return valid;

This code does generate unnecessary checks because another_check it still called even if first_check returned false.

We should use operator&& instead to get the intended short-circuit evaluation behaviour.

[jcoupey]

From the tests I've been running based on #294, reduction in total solving time is somewhere around 5%, depending on problem type, number of threads and exploration level.