libsnapshot: stride compression

Alternate dispatching blocks between threads rather than splitting the
data beforehand and then sending to threads in order to ensure that
single threading + multithreading chunks data at the same locations.
Without this change, the resulting op count + data section of the cow
will differ between --enable-threading && --disable-threading at
runtime, which is a result we don't want

Test: th
Change-Id: I3ed8add0552745a281fce2aa7f1d1d32eb547e63