We want to detect a situation where task in fiber is too eager for
stack memory and relax rss usage in such case. For this sake upon
stack creation we put 8 marks near 64K bound (such params allows us
to fill ~1/4 of a page, which seem reasonable but we might change
this params with time).

Once stack get recycled we investigate the marks and if they were
overwritten we drop all pages behind to relax memory usage (if OS
supports madvise syscall).

Another important moment is that we're marking the whole stack
as not present thus if fiber never stepped over 64K limit the
marks will be in tact and it means the fibers are light ones
there won't be much #pf in future.

Later we plan to implement an intelligent fiber scheduling
considering how many memory fibers consume in average.

@locker:
 - fix watermark page alignment for grow-up stack
 - improve MADV_DONTNEED check
 - clean up code and elaborate comments
 - add test case to unit/fiber
 - fix unit/guard test

Follow-up #3418