Decouple client cot object from transmission tak_proto version

[sei-jmattson]

As a developer, I'd like to build COT event objects as either protobuf or XML and have pytak convert as necessary to the desired tak_protocol bytes for transmission.

This allows folks to develop withtakproto but still be able to function in TAK_PROTO=0 environments.
This allows mixing of new code producing protobuf event objects and legacy code with ET events in the same client.
Likewise, it would allow receipt of both TAK_PROTO 0 and 1 messages at the same time (which seems to be how ATAK is functioning).
 
If you'd be amenable to a PR implementing such, I'm thinking along these lines:

• takproto.proto2xml() to complement the existing takproto.xml2proto()
• Have TXWorker determine type of dequeued object (CotEvent or ET) and serialize according to the configured TAK_PROTO option
• Have RXWorker deserialize according to nature of received bytes and convert (if necessary) to the TAK_PROTO option

[ampledata]

@sei-jmattson PR welcome!