perf(hogql): Fix C++ parser leaks

[Twixes]

Problem

There were some memory leaks left in #17659.

Here's how the Python parser behaves:

And here's how the C++ one used to:

Changes

There were two classes of memory leaks I identified:

1. PyObject*s that weren't decremented. I knew about when merging perf: Parse HogQL with C++ for a huge speedup #17659, just left that for later.
2. ANTLR C++ objects that weren't deleted. I thought the parser class destructor handled this, but it didn't.

Parsing looks like this now:

How did you test this code?

Added MemoryLeakTestMixin that does what it says on the tin. Half of the TestParser tests fail with that mixin on master, all pass with the fixes here.

[mariusandra]

This is lovely!