Test close methods in API

Various parts of the API have close methods (FluoClient, Transaction, Snapshot, etc). Need to add ITs that exercise these inorder to define behaviour and prevent regressions. At least need to test calling close multiple times and using object after close is called.

Start by inspecting the close methods on FluoClient, Transaction, Snapshot, and the other API objects named in the issue, along with the repository's existing integration-test conventions. Add integration tests for repeated close calls and for use after close, with expected behavior defined by the current API contracts; done means these cases are covered and the integration tests pass.

Written by the indexing model from the issue text.