Submitted by: @dyemanov

Provide configuration options, both global (defined in firebird.conf) and per-database (defined in databases.conf), that can be used to set up the necessary caching and locking mode, corresponding to the priorly existing architectures.

Two new configuration options are introduced:

- SharedDatabase defines whether a database file is shared at the filesystem level, thus allowing independent connections (possibly running in different processes) to access a database. In the shared database mode, database I/O is synchronized via the lock manager. Otherwise, a database file is opened in the exclusive mode.

- SharedCache defines whether a database page cache is common for all connections running inside this server process.

Compatibility matrix:

- SharedDatabase = false and SharedCache = true means SuperServer
- SharedDatabase = true and SharedCache = false means Classic or SuperClassic
- SharedDatabase = false and SharedCache = false means single-user mode