Fix non-threadsafe hash-table accesses

Fix unsafe hash-table accesses (#189)

Fact: It's not safe to mutate aws_hash_table while holding an aws_hash_element.
Fact: `outstanding_requests_table` is accessed by multiple threads. We always hold a lock while accessing it.

Bug: There were several places where we release the lock, but continue to hold an aws_hash_element we got from a find() call. Once the lock is released another thread can mutate the `outstanding_requests_table`, leading to undefined behavior when the aws_hash_element is accessed again.