Structured debug events

[sisuresh]

Refactor of #640.

Add diagnostic ContractEvents that are not metered. I used ContractEvents because they are structured, allowing downstream consumers to parse a format they should already be familiar with if they consume regular events. These are not the same as the existing DebugEvents, which are non structured messages returned by the host. These diagnostic events are also returned by the host, but should be included in a non hashed portion of tx meta.

The associated core and xdr changes would look something like sisuresh/stellar-core@78d67fd and sisuresh/stellar-xdr-next@878cc77.

Note that -

1. InternalEventsBuffer::record needs metering added for non structured debug events.
2. Removes the mechanism that converts ContractEvents to DebugEvents when they're rolled back.
3. Stops rolling back objects.

[graydon]

Structurally seems ok, a handful of implementation nits (especially the back-and-forth between XDR and host domain objects/values).

[graydon]

@sisuresh resolved most conversations, thanks! a few tiny nits remain but none are blocking, I'll set review-approved and you can merge whenever based on your preference re: fixing them or just moving on :)

[graydon]

(I believe you can push the "resolve" button on those remaining conversations, if I understand GH policy correctly)