Fixes #1913

AuthorityFactory::createBetweenGeodeticCRSWithDatumBasedIntermediates() issued
a complex SQL query that pushes the SQLite3 query plan optimizer to its limits.
Was working reasonably with sqlite 3.11, but not with later versions.
So put less constraints in the main query and do post-processing checks and
auxiliary requests to avoid such issues.

For some unknown reason, this slightly slows down a bit execution time of the
whole test_cpp_api binary (~ 10%), but couldn't come with something better,
despite trying many variations of the main SQL query. It seems that in the
general case the non-filter LEFT JOIN on the supersession table helped,
except on this EPSG:7842 case.

Fixes #1867

As on import of EPSG, we remove the supersession of Canadian NTv1 file w.r.t NTv2
(because the default behaviour of PROJ is to ignore superseded operations). However
the NTv1 operation is advertized with an accuracy of 1m, whereas NTv2 is advertized
with 1.5m. Consequently on areas where both files are valid, and if both files are
available, NTv1 would be selected. So as a workaround, worsen the NTv1 accuracy to
2m so that NTv2 is used in priority.