[6.x] Fixed breaking compatability with multi-schema postgres

[raymondtri]

The Postgres engine supports a multi-schema search path, and parts of the PostgresBuilder do not.

This PR addresses multiple schema support in a non-breaking way for 6.x.

Furthermore, to give more granular control, I allowed the $excludedTables portion of the dropAllTables() method to be set via configuration values, while gracefully falling back to the original value.

This level of granularity is particularly important when it comes to postgresql extensions and the tables they sometimes create.

[driesvints]

Changing the method signature like this is a breaking change unfortunately and should target master instead.

[taylorotwell]

I think if you wanted to do this you would need to handle strings OR arrays.