Fixes segmentation fault in FUNC_API_Send (MAID-191)

When the test pc was under a heavy load, some tests from the FUNC_API_Send test group timed out. The previous code did not handle the case where the original async request did eventually get called after we've already received timeout so the next test received two async responses which tried to set std::future. This resulted in invalid memory read (reported by valgrind) which later caused segmentation faults.