Ref CID#469268

Motivated by #41

the disk object is always a multiple of 4K, and for higher values (like
12K), rounding up to the next power of two does not make sense.

So, just use two allocations for FCO and FDO always.

For now, this uses a local installation of coverity's tools in the slink
subfolder.

As the fdr argument is from fbo->region, we should DTRT for this
situation and not bloat the code with a loop and stuff...

Motivated by #39

The code accepted the right pointer, the right offset _OR_ the right
size. Which lead to the wrong (usually the last) segment being freed.

Fixes #39

... in fellow_busy_obj_trimstore():

The code did not work correctly when the region was to be removed
(because it was reduced to size zero) _and_ it was not the last of the
regions.

Part of the fix for #39

assert that the FCS_BUSY segment to be trimmed is in fact contained in
the current body region.

The previous code

-       if (fdsl->nsegs == fcsl->lsegs ||
-           fcsl->segs[fdsl->nsegs].state == FCS_INIT) {

was confusing, it relied on the fact that, when an fcsl has more
segments than an fdsl, the first "surplus" segment has state FCS_INIT,
when it makes much more sense to just check fdsl->nsegs against
fdsl->lsegs.

This concerns an open issue from before the public release, which I
had not understood before and believe to have understood now.

With this code *) added to the test

@@ -5946,6 +5949,15 @@ t_cache(unsigned chksum)
	test_bocdone(fbo, TRUST_ME(hash), 1);
	fellow_cache_obj_deref(fc, fco);

+	// === max out region alloc
+	fbo = fellow_busy_obj_alloc(fc, &fco, &priv2, 1234).r.ptr;
+	CHECK_OBJ_NOTNULL(fbo, FELLOW_BUSY_MAGIC);
+	for (u = 0; u < FCO_MAX_REGIONS; u++)
+		AN(fellow_busy_region_alloc(fbo, 1234, INT8_MAX));
+
+	test_bocdone(fbo, TRUST_ME(hash), 1);
+	fellow_cache_obj_delete(fc, fco, hash);
+
	// === alloc space, dont use

we tripped here

   assert(flivs->oob || u == obj_alive);

with u == 1 and obj_alive == 0.

So the offset of a region from a dead object was not taken by a
subsequent allocation, which is fine, why should it be?

*) Note: The added test code is not correct yet, as it does not
   register the regions with the segment list, so obj_delete leaks.

This change should be semantically equivalent, but Flexelint understands
it.

It was too complicated and limited by waiting for flushes to finish.

Now that we can issue multiple flushes, we can simplify it
substantially.

As a result from intermediate efforts, there is now also a facility to
base nuking on the amount of data currently in the process of freeing.
Leaving it in #ifdef'ed out in case we'll need it again.

with more than once flush finish, writing a header from an
old flush could race the logbuffer_ref() from a more recent one,
leading to an inconsistent log where a logblock with next_off == 0
became reachable.

To avoid having to wait for a previous flush to finish (in most cases),
we now allocate the flush finish state dynamically (and asynchronously).

For ordinary flushes, we can now start the next flush while a previous
one is still in flight, ordering the flush finish in a list to preserve
log consistency.

as there is only one thread waiting

the logwatcher has now been, for a long time, the only thread
waiting on it