VAULT-5827 Don't prepare SQL queries before executing them
We don't support proper prepared statements, i.e., preparing once and
executing many times since we do our own templating. So preparing our
queries does not really accomplish anything, and can have severe
performance impacts (see
https://github.com/hashicorp/vault-plugin-database-snowflake/issues/13
for example).
This behavior seems to have been copy-pasted for many years but not for
any particular reason that we have been able to find. First use was in
https://github.com/hashicorp/vault/pull/15
So here we switch to new methods suffixed with `Direct` to indicate
that they don't `Prepare` before running `Exec`, and switch everything
here to use those. We maintain the older methods with the existing
behavior (with `Prepare`) for backwards compatibility.
