decompressor: add TODO about perf implications of creating new DCtx
decompressor: add TODO about perf implications of creating new DCtx

Crude benchmarking shows that failure to reuse the DCtx makes
decompress() w/o a dictionary ~40 MB/s slower (206 vs 166MB/s) and
~200 MB/s with a dictionary (~536 vs ~322MB/s). That is statistically
significant.
terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard