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: #2578.

Examples:

• One striking example is TransactionProcessor that delegates all ingesting code to TransactionBatchInsertBuilder.
• Example fix (of OffersProcessor): services/horizon: Fix separation of concerns related to offers ingestion #3083.

List of processors to fix:

• OffersProcessor
• AccountDataProcessor: move XDR ingestion code to processor #3935
• AccountsProcessor: move XDR ingestion code to processor #3934
• AssetStatsProcessor
• ClaimableBalancesProcessor: move XDR ingestion code to processor #3936
• EffectProcessor
• LedgersProcessor
• OperationProcessor
• ParticipantsProcessor
• SignersProcessor
• TradeProcessor
• TransactionProcessor
• TrustLinesProcessor
• LiquidityPoolsProcessor