IMPORT: slz: implement a synchronous flush() operation

IMPORT: slz: implement a synchronous flush() operation

In some cases it may be desirable for latency reasons to forcefully
flush the queue even if it results in suboptimal compression. In our
case the queue might contain up to almost 4 bytes, which need an EOB
and a switch to literal mode, followed by 4 bytes to encode an empty
message. This means that each call can add 5 extra bytes in the ouput
stream. And the flush may also result in the header being produced for
the first time, which can amount to 2 or 10 bytes (zlib or gzip). In
the worst case, a total of 19 bytes may be emitted at once upon a flush
with 31 pending bits and a gzip header.

This is libslz upstream commit cf8c4668e4b4216e930b56338847d8d46a6bfda9.