Like ZSTD_initCStream_usingDict(),
ZSTDMT_initCStream_usingDict() now keep a copy of dict internally.
This way, dict can be released :
it does not longer have to outlive all future compression sessions.
Correctly compress with custom params and dictionary
Added relevant fuzzer test in zstreamtest
Also :
new macro ZSTDMT_SECTION_LOGSIZE_MIN, which sets a minimum size for a full job
(note : a flush() command can still generate a partial job anytime)
Since the result of mt compression is a single frame,
changed naming, which implied the concatenation of multiple frames.
minor : ensures that content size is written in header
