Track memory of RAM filesystem

[simongog]

The in-memory construction provided by method construct_im calls the usual semi-external construction but uses a simple filesystem in RAM instead of disk files. Method construct_im is only intended for very small inputs (at most a couple of kBs) where construction is faster than interacting with disk.
The memory of the RAM filesystem was not yet tracked by the memory_monitor. This branch fixes this issue.

[tb-robot]

Can one of the admins verify this patch?

[mpetri]

can we use this new track_allocator to also keep track of the currently untracked memory (m_longsuperblock and m_miniblock) in select_support_mcl?