Support a single key propagation mode.

[rnburn]

Adds an option use_single_key_propagation to the LightStep tracer. When set, the tracer will propagate context using a single key with a base64-encoding of a binary representation of the span context. The goal is to eventually move this to some sort of standardized header key, but as an interim step it uses the x-ot-span-context that Envoy is using.

Also,

• Adds in the 3rd party code (from envoy) to handle the base64 encoding/decoding.
• Reduces string copying in the propagation code by replacing std::istringstream's with a non-copying variant.
• Uses the Carrier::LookupKey function if available for faster propagation.

[jmacd]

Looks really good