Add custom EXI buffer management

Add custom EXI buffer management

Exificient's SAXDecoder by default does not free its internal buffer,
and allocates one for each decoder. With version 1.0.4 we have enough
visibility to perform buffer management.

This adds a thread-local buffer, which is reused during parse operation
and retained if it is under 64kB after the operation completes.

Change-Id: Ie2f7c72b1160d389a07c473fda6739b7eb7212cb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 791a6a25f6670a23d390bfdeb786f70588d622a5)