null contradicts the phpdoc

When the connection is lost or is closed, subsequent transaction will no longer be nested because they started in a brand new session. Our internal representation of the nesting shold take this into account

using params in query cache key generation.

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.