Use a single JDBC layer per read/write

[jeremyprime]

Summary

Use a single JDBC layer, which holds the actual connection, for read or write operations.

Description

We create a new JDBC layer, and thus a new connection, each time a read or write pipe is called for. This results in additional connections at runtime, some of which are not closed when the operation completes. This fix recreates the JDBC layer each time it is called, if the underlying connection was closed.

Further work is still needed to avoid opening and closing multiple connections, which will be performed under another ticket.

Related Issue

Closes #314.

Additional Reviewers

@alexey-temnikov
@alexr-bq
@Aryex
@jonathanl-bq

[jeremyprime]

I was unable to create a singleton of the connection or JDBC layer due to our use of a lazy connection and the fact the JDBC layer may contain a closed connection (cannot be reopened, must create a new one).

These vars hold a single JDBC layer per operation (read and write), and the logic below will recreate them (and their underlying connection) if the connection was closed.

In the future we should properly refactor our use of the JDBC layer and connection to avoid closing and opening connections unnecessarily (will create a separate ticket for that).