Footgun with the `compressobj()` API

The fact that you can create multiple compressobj from a single ZstdCompressor allows for foot-guns such as:

a = zstandard.ZstdCompressor()

b = a.compressobj()
c = b.compress(b"prefix")

d = a.compressobj()
e = d.compress(b"foo")
e += d.flush(zstandard.FLUSH_BLOCK)

c = b.compress(b"foo")
c += b.flush(zstandard.FLUSH_BLOCK)

assert(c != e) # Assertion fails!

The API should protect users from interleaving usages of two compressobj. This could be accomplished via a counter that is atomically incremented in the ZstdCompressor. The compressobj would know what count it was created on and throw an error if compress() or flush() are called after the counter in the parent ZstdCompressor is incremented.

Thanks @thatch for identifying this issue and proposing the fix.

Start by reproducing the interleaved compressobj() calls in the issue, then inspect the ZstdCompressor and compressobj implementation. Done means later operations on an older compressobj are rejected after another object is created, with the reported assertion no longer failing.