[rpc] Deprecate estimatefee RPC

Deprecate estimatefee in v0.16, for final removal in v0.17.

This commit introduces a phased removal of RPC methods. RPC method is
disabled by default in version x, but can be enabled by using the
`-deprecatedrpc=<method>` argument. RPC method is removed entirely in
version (x+1).

src/rpc/mining.cpp

@@ -789,6 +789,12 @@ UniValue estimatefee(const JSONRPCRequest& request)
             + HelpExampleCli("estimatefee", "6")
             );
 
+    if (!IsDeprecatedRPCEnabled("estimatefee")) {
+        throw JSONRPCError(RPC_METHOD_DEPRECATED, "estimatefee is deprecated and will be fully removed in v0.17. "
+            "To use estimatefee in v0.16, restart bitcoind with -deprecatedrpc=estimatefee.\n"
+            "Projects should transition to using estimatesmartfee before upgrading to v0.17");
+    }
+
     RPCTypeCheck(request.params, {UniValue::VNUM});
 
     int nBlocks = request.params[0].get_int();