Add implementation with advisory(session) lock

[nkonev]

See https://github.com/blagerweij/liquibase-sessionlock

[nkonev]

Upd:

Starting from 2.9.0 so-called database-specific lock was implemented.
It is advisory lock in case PostgreSQL, session lock in case MySQL and MariaDB. For other databases it is (still) usual table-based lock.
This means, if you migrating from 2.8.x in case before mentioned databases you can remove your lock table by

drop table migrations_lock;

If you want to preserve old behavior - you can set

r2dbc:
  migrate:
    prefer-db-specific-lock: false

[nkonev]

SQL Server and H2 aren't seen very popular, leaving them with table locking for now, session lock can be implemented in the future or by demand.