asset-s/QuoteRequestor: info log Objects, not strs

In the QuoteRequestor class, when _infoLogger() is used, log objects
directly, rather than stringifying them first.  This changes the 0x API
log rendering from

{"level":"info","time":1588209135290,"pid":13022,"hostname":"precision5510","msg":"{\"aggregatedRfqtLatencyMs\":4}","v":1}

to

{"level":"info","time":1588224481908,"pid":3637,"hostname":"precision5510","aggregatedRfqtLatencyMs":2,"v":1}

This facilitates parsing of these logs by Kibana.

packages/asset-swapper/src/utils/quote_requestor.ts

@@ -86,10 +86,8 @@ export class QuoteRequestor {
 
     constructor(
         private readonly _rfqtAssetOfferings: RfqtMakerAssetOfferings,
-        private readonly _warningLogger: (s: string) => void = s => logUtils.warn(s),
-        private readonly _infoLogger: (s: string) => void = () => {
-            return;
-        },
+        private readonly _warningLogger: (a: any) => void = a => logUtils.warn(a),
+        private readonly _infoLogger: (a: any) => void = () => undefined,
         private readonly _expiryBufferMs: number = constants.DEFAULT_SWAP_QUOTER_OPTS.expiryBufferMs,
     ) {}
 
@@ -131,7 +129,7 @@ export class QuoteRequestor {
                 return undefined;
             }),
         );
-        this._infoLogger(JSON.stringify({ aggregatedRfqtLatencyMs: Date.now() - timeBeforeAwait }));
+        this._infoLogger({ aggregatedRfqtLatencyMs: Date.now() - timeBeforeAwait });
 
         const responses = responsesIfDefined.filter(
             (respIfDefd): respIfDefd is AxiosResponse<SignedOrder> => respIfDefd !== undefined,
@@ -229,7 +227,7 @@ export class QuoteRequestor {
                 return undefined;
             }),
         );
-        this._infoLogger(JSON.stringify({ aggregatedRfqtLatencyMs: Date.now() - timeBeforeAwait }));
+        this._infoLogger({ aggregatedRfqtLatencyMs: Date.now() - timeBeforeAwait });
 
         const axiosResponses = axiosResponsesIfDefined.filter(
             (respIfDefd): respIfDefd is AxiosResponse<RfqtIndicativeQuoteResponse> => respIfDefd !== undefined,