Fuzzer: Add a flag to control the hang limit

[kripken]

Setting it to 0 disables hang limit checks, which is nice for fuzzers that
have their own timeout handling anyhow.

To expose this in a nice way, add a setFuzzParams() method on the
fuzzer. And to make that simple, remove some ancient
"FuzzParamContext" logic that was apparently an idea to have
stacks of fuzz params. We never implemented that, so it seems better
to remove all that complexity.

[kripken]

cc @Liedtke