re-generate messages file

[bghitulescu]

We started seeing a log of errors like this

/usr/local/lib/python3.8/site-packages/photonpump/messages_pb2.py:538: DeprecationWarning: Call to deprecated create function FieldDescriptor(). Note: Create unlinked descriptors is going to go away. Please use get/find descriptors from generated code or query the descriptor_pool.
eventconsumer_1  |   _descriptor.FieldDescriptor(
eventconsumer_1  | /usr/local/lib/python3.8/site-packages/photonpump/messages_pb2.py:555: DeprecationWarning: Call to deprecated create function FieldDescriptor(). Note: Create unlinked descriptors is going to go away. Please use get/find descriptors from generated code or query the descriptor_pool.
eventconsumer_1  |   _descriptor.FieldDescriptor(
eventconsumer_1  | /usr/local/lib/python3.8/site-packages/photonpump/messages_pb2.py:572: DeprecationWarning: Call to deprecated create function FieldDescriptor(). Note: Create unlinked descriptors is going to go away. Please use get/find descriptors from generated code or query the descriptor_pool.

Currently photon-pump has a dependency on protobuf (protobuf>=3.5.1). When installing photon pump, the latest protobuf is installed (currently 3.12.2) and these errors start to popup. In order to fix this, this PR:

• updates the minimum accepted version of protobuf going forward to 3.12.0
• updates the messages_pb2.py module (generated using the protocol buffer compiler)

[hjwp]

@epoplavskis shall we merge and do a release?

[epoplavskis]

Yep, go for it