Change the page cache so that a new sqlite3_pcache object is allocated as
soon as the page cache is opened, not delayed until the first fetch request.
This give a noticable performance boost.  The interface between pager and
the page cache has changed slightly, which might break ZIPVFS.
Performance enhancement in sqlite3VdbeMemNulTerminate().
Factor out the exception paths from sqlite3ValueToText() into a separate
function so that the main routine is much faster for the common case of
no required type or encoding conversions.