Implementation of isValid property

[gsbelarus]

IsValid() function implemented for objects Client, Attachment, Transaction, Statement, ResultSet, BlobStream. So, we can test the object's validity after commit(), rollback(), dispose(), close() etc. methods have been invoked.

The newly added method is covered through the tests. In two places the tests have been adapted to be able to run on FB3.

Example of usage:

const transaction = await attachment.startTransaction();
try {
  ...
  // some code where the transaction could be committed or rolled back
  // but there are also conditions for the transaction to stay alive
  // until the code ends
  ...
} finally {
  if (transaction.isValid()) await transaction.commit();
}

[asfernandes]

Please add it to Blob and Events.

[asfernandes]

Please replace isValid to be a readonly property instead of a method. It looks more friendly.

[gsbelarus]

resolved all issues mentioned above.

[asfernandes]

Please set option "allow edits from maintainers" in this PR so I can send small edits before merge.

[gsbelarus]

It is set.

[asfernandes]

It is set.

I still cannot push to it.

[asfernandes]

I have manually rebased your changes with mine and pushed to master, so closing this.