Allow individual database configuration

[evgeniy-scherbina]

PR allows individual database configuration

app.toml example:

[rocksdb]
enable-metrics = true
report-metrics-interval-secs = 15
max-open-files = 16384
...

[rocksdb.blockstore]
max-open-files = -1
block_size = 4096

[rocksdb.tx_index]
max-open-files = -1
block_size = 4096

[rocksdb] section contains default/fallback database configuration, but you can override it with database-specific configuration

it's very convenient approach, because rocksdb configuration is pretty extensive, contains around 30 parameters, but usually we override only few of them

so we can define standard configuration in [rocksdb] section, and then override only few params in database-specific configurations