Tracker Initialization: allow to re-initialize

[sdirosa]

Currently Trackers are immutable, exception made for a few properties like the active flag.

Nonetheless, sometimes it's necessary to reconfigure a Tracker instance with more complicated changes, like a new set of plugins, or a different Transport.

We have built a PoC for this in the https://github.com/objectiv/objectiv-analytics/pull/1164 PR, and we need to formalize these changes in a more coherent set of changes:

• Plugins and Transport should not rely on constructors to initialize, instead we should have a dedicated lifecycle method
  • Plugins already have initialize, so we have to check if all of them are using that exclusively
  • TransportInterface needs an initialize method, then we have to move constructor logic in there
  • Both Plugins and Transports should be stateless, so we may re-initialize them safely
• Trackers should not perform initialization in their constructor, instead there should be a method for it
  • As for Plugins and Transports, the Tracker should manage its state so it can be safely re-initialized with a new config