Abstract out the dependency between B-Tree layer and the sqlite3 structure.

This is a correct analysis from very outdated documentation... The sqlite3 structure is used for:

The unlock-notify feature (arguments to sqlite3ConnectionBlocked() are database handles),
Accessing the SQLITE_ReadUncommitted flag,
Invoking the busy-handler callback,
During sqlite3BtreeOpen(), to find the VFS to use,
Accessing the SQLITE_SharedCache flag (for setting it),
To check the same B-Tree is not attached more than once in shared-cache mode,
To link the B-Tree into the pointer-order list of shared-cache b-trees used by the same handle (used for mutexes).
To determine if an in-memory sub-journal should be used.
To know how many savepoints are open in BtreeBeginTrans().
Many, many times to assert() that the db mutex is held when the b-tree layer is accessed..

LumoSQL / archive1-LumoSQL-on-github