Rationalize the usage of `Connection` in the JDBC appender

The JDBC appender takes a very conservative approach to SQL connection sharing:

1. as most appenders, each `append` call is synchronized,
2. each event (or batch of events) is logged using a different `Connection` object, which is closed at the end of the call,
3. to prevent the high performance hit from creating many `Connection`s, users are advised to use a connection pool like DBCP2.

This approach has many disadvantages:

- it does not profit from the connection pool, since at any time at **most** one connection will be borrowed from the pool,
- on the other hand users that use `DriverManager` get a performance hit, even if 1. ensures that no `Connection` object will be used concurrently on multiple threads.

Start by locating the JDBC appender and reading its append and connection-lifecycle logic. Investigate the stated trade-offs between synchronized appends, connection pooling, and DriverManager usage; done should establish a decided connection-sharing approach and verify its behavior under the described usage patterns.

Written by the indexing model from the issue text.