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.

params added to aws_mqtt_client_publish_received_fn()

• API CHANGE: dup, qos, and retain, params added to aws_mqtt_client_publish_received_fn()

Symbol exports and some in-progress ffeatures

• Symbol export fixes
• some in-progress mqtt features

Update to new host resolver creation interface

• updates tests to use the new default host resolver creation function

Swtiching to Disconnect before callback

• Before invoking callback like on_completed/on_disconnected, the state should be switched to disconnected., so that user can call functions from callback as they want