Transaction meta is missing one time signer changes in older versions of the protocol

[MonsieurNicolas]

When removeUsedOneTimeSignerKeys was added in
840787d#diff-94d5ac6726dabe26918718d13fd44b9cR540

the meta format was not updated to reflect those changes (this would have required a "post transaction" meta of sorts).

This was later fixed in protocol 10 (presigned transactions changes are tracked in txChanges as they are done before applying operations):
#1464 (comment)

Proposed fix:
in order to generate meta that contains all changes, a simple update to pre-protocol 10 ledgers would be to modify the meta of the last operation to include the one time signer change (NB: this is a merge of 2 meta).
It's a bit ugly as technically speaking the change doesn't belong to the last operation, but it seems to be the simplest we can do without adding "post tx" meta.

[MonsieurNicolas]

Tagging as "discussion" for now. @jonjove FYI

[jonjove]

I think the best approach is to update the XDR with

struct TransactionMetaV2
{
    LedgerEntryChanges txChangesBefore;
    OperationMeta operations<>;
    LedgerEntryChanges txChangesAfter;
};

union TransactionMeta switch (int v)
{
case 0:
    OperationMeta operations<>;
case 1:
    TransactionMetaV1 v1;
case 2:
    TransactionMetaV2 v2;
};

The contents of TransactionMetaV2.txChangesBefore and TransactionMetaV2.operations would be identical to the contents of TransactionMetaV1.txChanges and TransactionMetaV1.operations. TransactionMetaV2.txChangesAfter would contain the changes for the one-time signer changes up to version 9, and would be empty in versions 10 and beyond.

In order to avoid breaking downstream systems, we would also add a configuration flag SUPPORTED_META_VERSION with default value 1 and the only other acceptable value 2. If SUPPORTED_META_VERSION = 1 then stellar-core emits TransactionMetaV1 and analogously if SUPPORTED_META_VERSION = 2 then stellar-core emits TransactionMetaV2. The intention would be to deprecate generation of TransactionMetaV1 in a few (perhaps 3?) months, so we should also emit a deprecation warning on start-up if SUPPORTED_META_VERSION = 1.

[MonsieurNicolas]

@ire-and-curses FYI