Remove run_on_reactor (#3395)

synapse/federation/transaction_queue.py

synapse/handlers/auth.py

synapse/handlers/federation.py

synapse/handlers/identity.py

synapse/handlers/message.py

synapse/handlers/register.py

synapse/push/pusherpool.py

synapse/rest/client/v1/register.py

synapse/rest/client/v2_alpha/account.py

synapse/rest/client/v2_alpha/register.py

synapse/util/async.py

tests/test_distributor.py

tests/util/caches/test_descriptors.py