Expressed PARAM_*_ARRAY through other constants to eliminate the dependency on the actual constant values

statements in a rewindable iterator. Skipped the test for iterating
multiple times with a message regarding that iterators are non
rewindable.

Changed DBALException::invalidPlatformSpecified() exception to be more detailed about what was given.

In a few cases methods in Connection may throw a DBALException which
isn't declared in the docblock. As the Connection class is heavily
used by third party code, this code runs the risk of ignoring
those exceptions instead of handling them. While it is
technically not required to declare exceptions in PHP, it is highly
recommended, especially in public APIs.

If you pass a null to the where array of update or delete it's
pretty obvious what you want.

Most (all?) databases won't match anything if you actually try
to compare with NULL making this a silent failure without it.

The param comment is currently set to string, which is wrong.
The correct hint would be integer/int

Improve return type's phpdoc because it wasn't clear from the phpdoc or the online documentation what would happen if no row was found.

it is best practice to implement retry logic for transactions that are aborted because of deadlocks or timeouts. this makes such method available inside the DBAL and also add detection for errors where retrying makes sense in the different database drivers

Fixes an issue when calling `ping()` where `$this->platform` is null. Happened to me because using the `'pdo'` param bypasses the `connect()` method and the platform is never set without explicitly calling the method.

Not sure if this is the most elegant way to deal with this but it worked enough for my purposes so I figured I should submit a PR.

These events don't exist.