Polishing the basic Bitcoin example

[THLO]

The PR polishes and fixes a few minor things in the basic Bitcoin example:

• Renamed variables to indicate that fees are measured in sat/vbyte (and not sat/byte).
• Introduced a transaction_id type as the return value for the send function.
• Introduced the SIGN_WITH_ECDSA_COST_CYCLES constant.
• Fixed the index for the median fee (the median fee is at index 50 and not 49).
• Reshuffled some comments and code.