Add no lock parameter for Postgres

[prika148]

Essentially a copy of Mysql no-lock parameter, but for Postgres.
Opened issue addressing this: #1001

Most changes were found in abandoned PR: #1030

[coveralls]

coverage: 56.384% (+0.07%) from 56.314%
when pulling 5d3e16c on prika148:prika148/add-no-lock-postgres
into 2788339 on golang-migrate:master.

[sid-maddy]

@prika148, could you possibly also add support for the pgx drivers—pgx:// and pgx5:// (pgx/v5)?