Improve support of Npgsql 4

[hazzik]

We have some compatibility issues with the newer driver, so need to upgrade

[hazzik]

Most likely changes required are similar to nhibernate/nhibernate-envers#2

[fredericDelaporte]

So, Npgsql 4 still has issues with connections released from the second phase of a system transaction commit/rollback.
As explained here, the TeamCity build agent may require some manual actions for allowing PostgreSQL other builds to succeed.

The 3.2.5 failure may lead to pending prepared transaction on PostgreSQL db, causing it to prevent nhibernate database drop, wrecking all subsequent builds whatever the PR. It is then required to use the pgadmin tool on build agent for rollbacking pending prepared transactions (see here) and dropping the db.

As suggested here, this PR may have to introduce a new (TestDialect?) property for ignoring tests involving connections used in commit phases. Then something like the following could be added into SystemTransactionFixtureBase:

protected override bool AppliesTo(Dialect.Dialect dialect)
	=> (TestDialect.SupportsConnectionUseOnSystemTransactionPrepare ||
			!UseConnectionOnSystemTransactionPrepare) &&
		base.AppliesTo(dialect);

[fredericDelaporte]

I have a bit mixed feelings about this change.

On the one hand, we already do this on Get, so doing this on Set is more consistent.

On the other hand, adding our own conversion here while all the other data providers do no require it and handles it themselves is likely to be an overhead for those other data-providers. They will still check the value type, and in some case even re-convert it. (That will be the case for Oracle with current NHibernate Double registration, which actually maps it to a numeric (or double precision which translates to numeric) instead of binary_double available since 10g.)

[hazzik]

I think for the most of the providers this will be no-op as types do typically match to the expected type. Also, most of the types already do some conversation on set, so it is not a big deal.