Integrate ldm with zstdmt

Integrate ldm with zstdmt

Integrate ldm into zstdmt by running it in serial and in order in the first
step of each job, in the same place as the hash gets updated. The input
buffer is sized to fit the whole LDM window and 2 full buffers of slack.
Input buffers cannot be reused until the LDM step is done with them.
After the LDM step is finished, the jobs don't actually have access to the
full window, only the overlap.

Tested on a few different multi-GB files with and without sanitizers,
and with different numbers of threads.