Stability fix

• Fixes a crash due to a race condition in the mqtt connection logic

Allow for null on-message callbacks in subscribe

• Fixes a crash when the on-message callback is null for a subscription
• Updates the public API documentation with some missing parameters and annotations as to what parameters can be null

Data race fixes

• Fixes several data races related to the outstanding requests table and a request's timeout task.

Fix non-threadsafe hash-table accesses (take 2)

v0.7.5

fix test function (#190)

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.

Direct mqtt through http proxy

• Adds support for establishing direct mqtt connections through an http proxy

publish payload

• We now keep a deep copy of the publish payload. User don't need to keep the payload alive for the whole process.

Proxy refactor

• Companion release to the new proxy systems in aws-c-http v0.6.2

Timeout configuration for requests

• API CHANGE: Added protocol_operation_timeout_ms to aws_mqtt_connection_options, which will be applied to timeout when waiting for the response to some operation (PUBLISH QoS>0 and UNSUBSCRIBE) requires response by protocol.

New managed thread API

Convert to new managed thread system for threads that cannot be reliably explicitly joined.