Manually merge 3.0.x into master

[morozov]

Q	A
Type	improvement
BC Break	yes (tiny)

This was quite an adventure because 2.10.x and master have some differences in static analysis configuration:

1. The master branch runs PHPStan against the tests/ directory while 2.10.x doesn't.
2. The PHP_CodeSniffer configuration in master requires language-level type declarations where possible while 2.10.x doesn't.

So the newly found issues had to be fixed right before or withing the merge:

1. The signature of MasterSlaveConnection::connect() was changed from ($connection = null) to (?string $connection = null). The reason is that Upgrade to PHPStan v0.12 #3799 removed the {@inheritDoc} annotation from the method which made PHP_CodeSniffer spot the missing type declaration that should have been added in Enforced parameter and return value types in Connection classes #3568.
2. Some unreachable code has been removed from tests since it's no longer allowed by PHPStan.
3. Unconditionally skipped tests were enabled (if they passed) or removed (if they failed).
4. A couple of tests got new assertions added to clarify the value types before using them.
5. Some existing issues found by PHPStan have been temporarily suppressed in the configuration and will be fixed later.
6. Some false-positives have been also suppressed and reported upstream (When analyzing usage of array_filter, ARRAY_FILTER_USE_KEY is not taken into account phpstan/phpstan#3132, PHPStan mistakenly assumes that an array can pass the is_numeric() test phpstan/phpstan#3133).