since these methods called via `to_sql` method under `unprepared_statement`

Fix rsim#1678 Backports rsim#1713 to release52 branch

Starting from Oracle enhanced adapter 5.2.0.beta1 most of the queries for the dictionary, such as all_sequences use bind variables by rsim#1498 .
It should help shared memory usage however, which eventually means Oracle enhanced adapter 5.2+ only supports `prepared_statements: true`
as Rails default configuration but it does not support `prepared_statements: false`.

Even if `prepared_statements: true`, `to_sql` uses `unprepared_statement` to generate SQL statement. If the connection already knows which table is associated with which model object in advance, `to_sql` works fine but when `to_sql` executed first the connection needs to know these relation then call
column_definitions` and `pk_and_sequence_for` methods at least.

it actually reverts rsim#1503

rsim#1498 implemented prepared statements for dictionary
queries. However rsim#1713 restored non-prepared statements
for `column_definitions` and `pk_and_sequence_for`, which likely have caused
longer elapsed time for these queries reported at rsim#1720

Those who needs 'cursor_sharing = exact' can specify its own value to .database.yml

it actually reverts rsim#1503

rsim#1498 implemented prepared statements for dictionary
queries. However rsim#1713 restored non-prepared statements
for `column_definitions` and `pk_and_sequence_for`, which likely have caused
longer elapsed time for these queries reported at rsim#1720

Those who needs 'cursor_sharing = exact' can specify its own value to .database.yml