Fix lockmode initialization for custom relation options
The code was enforcing AccessExclusiveLock for all custom relation
options, which is incorrect as the APIs allow a custom lock level to be
set.
While on it, fix a couple of inconsistencies in the tests and the README
of dummy_index_am.
Oversights in commit
773df88.
Discussion: https://postgr.es/m/
20190925234152.GA2115@paquier.xyz