Ensure tx meta is v2 when getting ledgers with protocol version <10

[bartekn]

As explained by @MonsieurNicolas in #1902:

There is some missing meta in protocol version older than version 10. This impacts transactions with one time signers (that get removed after operations in v9 and earlier; and during signature verification in v10 and later). Those changes are invisible in the meta which may confuse certain ingestion processors that keep track of signers on accounts.

It's possible that users will use stellar-core without SUPPORTED_META_VERSION=2 and in such case ingestion system can process meta incorrectly. I think we should check the protocol version of the ledger in DatabaseBackend.GetLedger and if it's <10 it should check if txmeta version is equal 2. Otherwise it should return error.

[bartekn]

Fixed in #2099.