Option to skip locking for postgres

[iavael]

Is your feature request related to a problem? Please describe.
In some postgresql setups (e.g. pgbouncer in transaction-pooling mode) session level features like prepared statements or session-level advisory locks are unavailable. Because of that migrate doesn't work in such setups.

Describe the solution you'd like
Add parameter to disable locking for postgres

Describe alternatives you've considered
None

Additional context
As I can see option to skip lockin was already discussed in #215 and implemented in #439, but only for MySQL. This option is needed for PostgreSQL too.