small: introduce small/static

Before the patch Tarantool had a thread- and C-file- local array
of 4 static buffers, each 1028 bytes. It provided an API
tt_static_buf() allowing to return them one by one in a cycle.

Firstly, it consumed totally 200Kb of BSS memory in summary over
all C-files using these buffers. Obviously, it was a bug and was
not made intentionally. The buffers were supposed to be a one
process-global array.

Secondly, even if the bug above had been fixed somehow, sometimes
it would have been needed to obtain a bit bigger buffer. For
example, to store a UDP packet - ~1.5Kb.

This commit replaces these 4 buffers with small/ static allocator
which does basically the same, but in more granulated and
manoeuvrable way. This commit frees ~188Kb of BSS section.

A main motivation for this commit is a wish to use a single
global out-of-stack buffer to read UDP packets into it in the
SWIM library, and on the other hand do not pad out BSS section
with a new SWIM-special static buffer. Now SWIM uses stack for
this and in the incoming cryptography SWIM component it will need
more.