Revert "bsnmpclient(3): make it thread-safe"

This reverts commit 89e3d5671ba13dceca272d5b159c9bd805f3f504.

As pointed out, there are several problems with that commit:

1. The new semantics, while useful for clients where multiple threads use separate contexts, breaks clients which correctly share a single one
2. Change in semantics would require a library version bump
3. It doesn't build with GCC