Add fast-path to ConfigPreprocessor::replaceParams for strings without parameters

Summary:
**This is a semi-automated perf-finding. I don't think this has huge impact given that config reloads are likely not very frequent, but it does have the nice benefit of making CPU less spiky when they do happen. I am happy to abandon or push, whichever the team prefers. This is however quite simple for a little savings.**

Strobelight profiling of edgeray/originray shows `ConfigPreprocessor::replaceParams` consuming ~1.8% exclusive CPU during config reloads (https://fburl.com/scuba/strobelight_services/zg0oj1mq).

The `replaceParams` function processes every string value in the mcrouter config JSON, scanning character-by-character via `unescapeUntil` looking for `%param%` substitutions. Most config strings do not contain parameter references or escape sequences, but the function still processes them character-by-character.

This adds a fast-path check at the top: if the string contains neither `%` nor `\`, return it immediately. `StringPiece::find` uses `memchr` which is highly optimized (SIMD on x86), making this check much faster than the character-by-character loop.

Combined with `ConfigPreprocessor::Context::add` (1.0%) and `Context::~Context` (1.0%), config preprocessing accounts for ~3.8% of total CPU. This fast-path reduces the `replaceParams` contribution for strings without parameters.

Reviewed By: stuclar

Differential Revision: D95426937

fbshipit-source-id: 329428d92aa7cbf617d791edbc6b5207580ad65d

Author: Brian Ecker

mcrouter/lib/config/ConfigPreprocessor.cpp

@@ -1828,6 +1828,14 @@ void ConfigPreprocessor::addMacro(
 dynamic ConfigPreprocessor::replaceParams(
     StringPiece str,
     const Context& context) const {
+  // Fast path: if the string contains no '%' or '\' characters,
+  // there are no parameters to substitute and no escape sequences
+  // to process, so return the string as-is.
+  if (str.find('%') == StringPiece::npos &&
+      str.find('\\') == StringPiece::npos) {
+    return str;
+  }
+
   string buf;
   while (!str.empty()) {
     auto pos = unescapeUntil(str, buf, '%');