Add toggle for shared cache mode

[PierceNg]

SQLite docu: https://www.sqlite.org/sharedcache.html

[PierceNg]

I have added the toggle in branch pierce15.

Notes:

• 2019 consensus on the sqlite-users mailing list seems to be "don't use shared cache mode".
• With URI naming, shared cache mode is enabled with a URI parameter "cache=shared". Of course this requires the SQLite shared library to be built with URI naming enabled.