Add mp_snprint() for debug output

Needed for #1828

See #1826 "memory explosion on UPSERT"

See #1825

Conflicts:
	debian/changelog
	debian/control
	src/lib/small

This patch fixes TODO in vy_wait_checkpoint() - instead of looping over
all ranges until all of them have min_lsn > checkpoint_lsn, it now waits
on a condition which is signaled as soon as all in-memory indexes with
min_lsn <= checkpoint_lsn are dumped.

By promoting all ranges that need to be checkpointed to the top of the
dump heap, we can fold peek_checkpoint logic in peek_dump.

While we are at it, remove scheduler->last_checkpoint_lsn and
checkpoint_in_progress, as they are never used.

We need it to check if a range has to be checkpointed. Currently, we
iterate over all attached in-memory indexes to get it. This patch adds
range->min_lsn instead.

A patch by @Gerold103 (Vladislav Shpilevoy), rebased and fixed.

Fixes #1805 - error with incorrect iteration over 'future' tuples.

diag_set(ClientError, ...) is needed for net.box and box.error()
which should work without box.cfg{}

Fixes #1814

diag_set(ClientError, ...) is needed for net.box and box.error()
which should work without box.cfg{}

Fixes #1814

Fix #1808

vy_range_delete() returns -1 if it fails to close range file. Nobody
checks what it returns and there isn't much we can do in case of failure
anyway. So let's make the function always return void and print all
errors to the log.

If woken up by range changes, the scheduler will keep queueing tasks no
matter if there are workers available for processing them. This is
incorrect, because the overall situation may change by the time a worker
completes processing its task. This patch puts the scheduler to sleep if
all workers are busy.

Fixes #1801

Old range files are removed immediately after split since commit
3977c18f ("vinyl: remove old range file on compaction asap"), so there's
no need to run garbage collection after each checkpoint. It's enough to
call it once, right after a successful recovery.

Bring in latest patches from upstream v2.1.
Fix issues with fiber switching on arm.
Fix gh-1799.

All keys before the split key can cancel each other, in which case the
new run is going to be empty. We are not ready to handle empty runs, so
we should silently cancel range file creation in this case.

"Too many open files" is not easy to fix.