Make sure this deprecated API is not used.

We need purely RAII-style, and this is std::lock_guard.

Reduce overhead by eliminating MultipleOutputs::ResetReopen().

We're using std::chrono::steady_clock now.  No need to duplicate code.

.. and remove the obsolete method OpenFilter().

Convert assertion to runtime check; this assertion could fail when the
output was closed due to an error before CLOSE arrived.