This ticket is an umbrella for bugs in our transactions implementation that were found when writing a spec test runner as part of GODRIVER-251:

1. abortTransaction should ignore all server errors. Relevant section of the spec: "After the retryable write attempt, drivers MUST ignore all errors from the abortTransaction command"
2. In Operation.Execute, we use the operation's read preference for server selection but use the transaction's read preference when creating the wire message. This can cause a mismatch. For example, if a collection with read preference secondary is used to run a transaction with read preference primary, a secondary will be selected but $readPreference will be primary in the wire message, causing a NotMaster error. We should use the transaction's read pref for everything.