Fixed (and small) maximal size of return parameters

Currently return parameters have a hard-coded upper-bound of 1024. We might consider making this dynamic. The obvious way would be to have a getOutputLenX function for each function X with a variable length response (e.g., Invoke). Somewhat related also with Bug #177.

PS: the hardcoding is in ecc/enclave/enclave_stub.go as variable MAX_RESPONSET_SIZE (note typo! Also don't mistake it with MAX_OUTPUT_SIZE which seems dead code ..)