Lua json module used to have a global buffer for all encodings. It
was reused by each next encode().

This was not correct, because during encode() might happen a GC
step, which might call encode() again and spoil the global buffer.

The same problem was already fixed for the global static buffer in
scope of #5632. Similarly to that time, the patch makes Lua json
module use cord_ibuf to prevent "concurrent" usage of the buffer
data. The global buffer is deleted.

According to a few microbenchmarks it didn't affect the perf
anyhow.

Core part of the patch is strbuf changes. Firstly, its destruction
is now optional, cord_ibuf can free itself on a next yield.
Secondly, its reallocation algorithm is kept intact - ibuf is used
as an allocator, not as the buffer itself. This is done so as not
to be too intrusive in the third party module which might need an
upgrade to the upstream in the future.

Closes #6050