Fix preconditions in transactions

[thebrianchen]

Context: b/135709736
Currently, if you read a non-existent document, then modify the document twice in the same transaction, each Write will include a precondition that the document did not exist. The server will reject this sequence since each precondition is applied on top of preceding writes in the transaction.

Instead, for the first write to a given key, generate a precondition that takes into account the read time and existence of the document, as we do today. For subsequent writes to a given key, generate a general precondition that only preserves the semantics of the operation: set() and delete() use Precondition(none) and update() uses Precondition(exists).

• Update precondition logic to buganizer spec.
• Reorder public and private methods in transaction.ts.
• Add table tests for testing transaction combinations.

[wilhuff]

First pass.

[wilhuff]

LGTM. Looks great!

A few final nits and an optional change to move fully to async/await if you wanted.