throwing reader_concurrency_semaphore::consume() causes memory leaks

[denesb]

Said method can now throw std::bad_alloc since aab5954. All call-sites should have been adapted in the series introducing the throw, but some managed to slip through because the oom unit test didn't run in debug mode.
In particular, the mutation_fragment constructors (which call consume()) and the tracking allocator are vulnerable.

[avikivity]

No vulnerable branches, not backporting.