The new implementation fixes the following:
* We can track more than 100 scriptPubKeys
* We can obtain more than 1000 transactions per sync
* `TransactionDetails` for already-synced transactions are updated when
new scriptPubKeys are introduced (fixing the missing balance/coins
issue of supposedly tracked scriptPubKeys)
`RpcConfig` changes:
* Introduce `RpcSyncParams`.
* Remove `RpcConfig::skip_blocks` (this is replaced by
`RpcSyncParams::start_time`).