Avoid MimeType garbage creation

[bclozel]

While profiling Spring web applications (MVC and WebFlux), it seems that MimeType (through MediaType) is creating a significant amount of garbage that could be avoided.

• Caching the toString() result of MimeType instances, since it cannot change and this method is called many times on hot paths
• MediaType.parseMediaType is called multiple times, for each request; this could be backed by a simple LRU cache implementation to avoid re-parsing known media types
• MimeTypeUtils.parseMimeType parsing code can be improved; a draft implementation shows +40% throughput and -10% allocation.
• MimeTypeUtils and MediaType both have static sections where well-known types are parsed; changing those from APPLICATION_JSON = MimeType.valueOf(APPLICATION_JSON_VALUE); to APPLICATION_JSON = new MimeType("application", "json"); leads to a bit of duplication but consistently reduces garbage and CPU usage at startup time (approx. 10ms).