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.