Add a fastMode for the common case when there are no new migrations to run

[rbygrave]

  /**
   * Set true to enable fastMode. This will perform an initial check for the exact same number
   * of migrations and matching checksums without any locking. If anything does not match
   * then the normal migration is performed with appropriate locking etc.
   */
  public void setFastMode(boolean fastMode)

This can be used to speed up the common case that there are no actual migrations to run (all migrations match).

If there is the same number of migrations (no new ones, all checksums match) then it can stop there.

This is "fast" in that it avoids the checks when all the migrations match:

• create the db schema if required
• does the migration table exist
• lock the migration table