Support custom transactions

[hmdhk]

• Review _finish, end, done, finish methods in Transaction
• Refactor TransactionService, Transaction, Span
• Add e2e tests for using custom transactions (Accessing the current transaction across JS files)
• Refactor navigation timing marks so it can be add to any transaction
• A configuration option to include navigation timing on the first transaction, or not at all, also whether to send it automatically or not
• Write documentation about using the API as well as common pitfalls
  • Async tasks like fetch and the right time to end the transaction
  • There only can be on transaction at any time