Do not skip an inaccessible database at agent startup

[dbulashev]

No description provided.

[cherts]

I think we should make an additional option, like test_db_connection_on_startup = true
This way, by default, we test the database connection and do not break backward compatibility.
It is probably important for some users to terminate the service if the database is not available.
If we disable this option (test_db_connection_on_startup = false), then pgSCV will start even if the database is not available at startup.

[cherts]

I made a slight mistake in the logic of checking the connection to the database, the variable is logically called skip_conn_error_mode and by default it will be false