Redo/fix RpcBlockchain implementation

The new implementation fixes the following:
* We can track more than 100 scriptPubKeys
* We can obtain more than 1000 transactions per sync
* `stop_gap` is enforced to ensure no missing balances

`RpcConfig` changes:
* Introduce `RpcSyncParams`.
* Remove `RpcConfig::skip_blocks` (this is replaced by
  `RpcSyncParams::start_time`).

Author: evanlinjin

examples/rpcwallet.rs

@@ -103,7 +103,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         auth: bitcoind_auth,
         network: Network::Regtest,
         wallet_name,
-        skip_blocks: None,
+        sync_params: None,
     };
 
     // Use the above configuration to create a RPC blockchain backend