Replace burn_chain/burn chain with burnchain

[friedger]

There are a small number of places that uses burn_chain or burn chain instead of burnchain.

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[r0zar]

I'm attacking this one with small updates at a time to make sure the changes are easy to follow.

I've started by renaming one variable in the burnchain.rs file. Local variables seemed like a good place to start.

I'll continue working through the app with variables before changing any function names. Right now I'm not sure if that will cause any breaking changes downstream.

Any direction or feedback is welcome as I work though this maintenance task.

[igorsyl]

Thank you for taking on this! @jferrant would be a good reviewer for this!