[REFACTOR] Remove JDBC reference from executor code

[Aryex]

Descriptions

Executors are not expected to use jdbc connections. However, we currently are passing JdbcLayerInterface references to them. This creates a few issues:

1. [BUG] Connector write/read opens multiple connections #398 was caused by this and more bugs may come up in the future.
2. It is also misleading since executors should not need to make any jdbc connections at all.

Thus, we should refactor the code to remove jdbc references from being passed down to executors, keeping it clean and avoiding future bugs.

[jeremyprime]

See also #324 for a related bug on this issue (and #314 before that, which was resolved).

[alexey-temnikov]

Downgraded to Normal priority since there is no functional or performance impact.