SQLite improvements for Spatial Index better update detection

[romicadascalescu]

SQLite provider improvements:

• provider could not 'see' updates of geometries between connection, leading to have multiple Spatial Index out of sync. This has been fixed by add an interface (a few plug-in functions) into sqlite engine which offers access to the Spatial Index
• made some improvements in sqlite engine to avoid releasing the internal schema each time a connection opens or closes a certain file.
• since the schema is shared between different connection against the same file now Spatial Index will be the same shared, this way we avoid having multiple Spatial Indexes for different connections. This will help a lot Map Guide by not loading Spatial Indexes as many connections we have.
• added new tests with multiple connection writing on them (not in the same time) and making selects to see if changes are visible between connections. - removed hook functions which fired for every update in database to avoid performance problems.
• enabled some spatial multi-thread tests commented out because SQLite provider got out of sync Spatial Indexes between connections