Serialisation of committed transactions is unsafe

[jumaffre]

At it stands, when serialise() is called when a transaction is committed (tx.commit()), we assume that the serialisation cannot fail (or if it does, we throw a specific error that should be caught by the caller (i.e. frontend.h) to abort the processing of transactions immediately.

Instead, we should recover safely, e.g. rolling back the store to the before the transaction was committed and returning a specific error message.

[eddyashton]

Updating with some further discussion from today:

• A common case of this that we need to handle is when serialisation fails because the write set is too large. We can write a unit test to easily repro this, writing a custom type with a custom serialisation function trying to write huge quantities of data.
• Rolling back is hard once we've generated a new commit index. Alternatives are to kill the executing node with an 'abort', or to commit some special tombstone marker to the ledger while returning an error to the user.