Fix that persistent would override other database options.

Closes: #1546

Follow the suggestions of [draft-knodel-terminology-00](https://tools.ietf.org/id/draft-knodel-terminology-00.html)
and other language frameworks and replace the notion of master/slave
with a more neutral primary/secondary.

* Raise minimum PHP version to 7.0

* Remove tests pre-PHP 7.2

* Upgrade dev dependencies

* Ignore tests for deprecated class

* Add typehints; not touching public API

* Remove none-array replacements-param; old behaviour from pre-1.4 release

* Psalm fixes

* Add upgrade notes

* PSR-12

PSR-12 compliancy

Fixes for the remainder of lib/SimpleSAML

The use of count function is wrong and unnecessary. If the array is
empty the foreach loop will not run.

Bugfix. While SimpleSAML\Database::query() returns a PDOStatement, SimpleSAML\Database::exec() doesn't. Make both return an integer with the amount of rows affected. Add a return value to SimpleSAML_Metadata_MetadataStorageHandlerPdo::initDatabase().

Fix some more phpdoc issues with returned values. Add a getLastError() method to retrieve information about the last error occurred.

Fix build: given that username and password are optional for some PDO drivers, do not enforce them to be set in the configuration. Use null as a default value for them.