Sweep abandoned blocks in eager sweeping

[qinsoon]

This PR fixes part of the issues in #824:

• Remove the incorrect abandon_blocks() call in FreeListAllocator::reset() for eager sweeping. The incorrect call caused blocks to be pushed to the global abandoned list before they are swept locally by the allocator.
• Add global sweeping that sweeps the abandoned list.

[wks]

It looks good to me.

It looks like the current implementation prefers the C-style while loop over the idiomatic for loop over range. It's a minor style problem. We can refactor it later.

[qinsoon]

It looks good to me.

It looks like the current implementation prefers the C-style while loop over the idiomatic for loop over range. It's a minor style problem. We can refactor it later.

I changed a few occurrences of C-style loop to Rust for loop.