www: reduce fragmentation in /t/ and /T/ endpoints HEAD master

For giant threads with /t/ and /T/ endpoints, avoid generating a
large string with a medium lifetime for the thread skeleton
($ctx->{skel}).  Instead, make $ctx->{skel} an arrayref and use
it to store a bunch of smaller strings, instead.

While keeping many small strings is inefficient due to pointer
chasing; forcing a smaller distribution of sizes makes it easier
for the malloc implementation to organize and find small chunks
of memory instead of having to find (and hold) larger contiguous
chunks.  When a large string is created now, it's lifetime is
kept as short as possible to decrease its likelyhood of causing
fragmentation.

Preliminary testing shows this appears to reduce RSS by roughly
20-40% under both glibc malloc (using a tiny
MALLOC_MMAP_THRESHOLD_=67000) on 32-bit and jemalloc 5.2.1 on
64-bit with standard settings.

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