[leo_backend_db] [bitcask] Control the trigger of bitcask merge operation

[mocchira]

When the size of a bitcask file exceeds 2GB(default), Merge( have a same meaning with Compaction on LeoFS) is automatically triggered.. This might cause system load high, so we need to control that timing for safe.

[yosukehara]

Here is default configuration value of bitcask:

• https://github.com/leo-project/leo_backend_db/blob/develop/src/leo_backend_db.app.src#L32-L35