Rewrite/reorganize Transaction Cost article

[mDuo13]

The Transaction Cost article is a bit confusing and hard to follow. Partly this is because the way the transaction cost works is confusing but partly it's because the article was only updated piecemeal after the introduction of FeeEscalation and the transaction queue.

Reorganize the article to make it clear:

• When and how the cost applies
  • Clarify the difference between the load-based fee scaling and the open ledger fee scaling (FeeEscalation)
  • How both types of scaling work (e.g. how does the server decide when/how much to scale)
  • What happens to transactions that don't meet one or both requirements
• Why the cost exists
• What the cost is (base and special cases)
• Other details that may be surprising (e.g. the transaction cost can destroy XRP set aside for the reserve; the network always destroys the exact amount specified even if it's more than necessary)
• Other relevant information or links to it (e.g. Fee Voting)

[mDuo13]

Style note: we prefer the term "transaction cost" rather than "transaction fee" because people tend to assume that a "fee" is paid to a specific party (such as Ripple), which is not how the transaction cost works. (The transaction cost XRP is destroyed.) The word "cost" conveys this nuance a little better, but the API, source code, and some developers still use the older term "fee" in many cases.