Fix https://github.com/julianhyde/optiq/issues/175, "Modify Schema SPI to allow caching".

Added "cache" attribute of Schema JSON element; add method contentsHaveChangedSince to Schema SPI (breaking change, if you've written a schema that does not extend AbstractSchema); big rework of how OptiqSchema builds maps of tables, functions and sub-schemas and how it handles case-insensitive matches.
Tweak parallelism of optiq-plus test suite.