Fix separation of concerns in Horizon processors

[bartekn]

Currently, many Horizon processors are relying on db2/history package to do parts of ingestion. db2/history methods should be dumb: they should accept struct to insert/update in a DB and do nothing else. Doing this will make code and tests simpler and will allow writing fuzzers with a mocked DBs.

While working on this we can fix existing issues related to using maps instead of structs: https://github.com/stellar/go/issues/2578.

Examples:

• One striking example is TransactionProcessor that delegates all ingesting code to TransactionBatchInsertBuilder.
• Example fix (of OffersProcessor): https://github.com/stellar/go/pull/3083.

List of processors to fix:

• [x] OffersProcessor
• [x] https://github.com/stellar/go/issues/3935
• [x] https://github.com/stellar/go/issues/3934
• [ ] AssetStatsProcessor
• [x] https://github.com/stellar/go/issues/3936
• [ ] EffectProcessor
• [ ] LedgersProcessor
• [ ] OperationProcessor
• [ ] ParticipantsProcessor
• [ ] SignersProcessor
• [ ] TradeProcessor
• [ ] TransactionProcessor
• [x] TrustLinesProcessor
• [x] LiquidityPoolsProcessor