fix(iot): increment mqtt encoder semaphore value on dealloc

[atierian]

Issue #, if available:
#2016

Description of changes:
_encodeSemaphore in AWSMQTTEncoder is created with dispatch_semaphore_create(1). When _encodeSemaphore is released (AWSMQTTEncoder dealloc'd) with a value of 0, a crash occurs due to the value being less than the original value (1).

Here's where the crash is occurring in libdispatch.

void
_dispatch_semaphore_dispose(dispatch_object_t dou,
		DISPATCH_UNUSED bool *allow_free)
{
	dispatch_semaphore_t dsema = dou._dsema;

	if (dsema->dsema_value < dsema->dsema_orig) { /* <--- crash happens here in libdispatch */
		DISPATCH_CLIENT_CRASH(dsema->dsema_orig - dsema->dsema_value,
				"Semaphore object deallocated while in use"); 
	}

	_dispatch_sema4_dispose(&dsema->dsema_sema, _DSEMA4_POLICY_FIFO);
}

This change increments the _encodeSemphore value via dispatch_semaphore_signal(self.encodeSemaphore); in dealloc of AWSMQTTEncoder.

Check points:

• Added new tests to cover change, if needed
• All unit tests pass
• All integration tests pass
• Updated CHANGELOG.md
• Documentation update for the change if required
• PR title conforms to conventional commit style

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[brennanMKE]

This PR will be replaced by #4211 which eliminates the semaphores from the IoT client code.