[lib] Set ZSTD_c_stable{In,Out}Buffer in ZSTD_compress2()

[lib] Set ZSTD_c_stable{In,Out}Buffer in ZSTD_compress2()

Sets these parameters in ZSTD_compress2() then resets them to their
orignal values after the compression call.

An alternative design could be to add a flush mode `ZSTD_e_singlePass`
which implies `ZSTD_c_stable{In,Out}Buffer` but only for a single
compression call, by directly setting the applied parameters. I've opted
for the smaller change, but this is open for discussion.