HotRod decoder small performance improvements

I noticed some small inefficiencies in the flight recordings from the client-server dist read benchmarks:

• Intrinsics.string() allocates a temporary byte[], we could use ByteBuf.toString(start, length, Charset) instead (which reuses a thread-local buffer).
• For reading the cache name it would be even better to use ByteString and avoid the UTF8 decoding.
• MediaType.hashCode() allocates an iterator for the params map even though it's empty.
• JBossMarshallingTranscoder.transcode() is called twice for each requests, and even when there is no transcoding to perform it does a lot of String.equals() checks.
• CacheImpl.getCacheEntryAsync() allocates a new CompletableFuture via applyThen() just to change the return type, could do the same thing by casting to the erased type.
• EncoderCache.getCacheEntryAsync() could also avoid allocating a CompletableFuture when the read was synchronous.
• Encoder2x is stateless, and yet a new instance is created for each request.
• Encoder2x.writeHeader() looks up the cache info a second time, as most requests needed that info to execute the operation, plus one useless (I think) String.equals() check for the counter cache.

There are also a few issues with the benchmark itself:

• The load stage took less than 3 mins according to the logs, but flight recordings show {{PutKeyValueCommand}}s being executed at least 1 minute after the end of the load phase.
• Either RadarGun or FlightRecorder itself is doing lots of JMX calls that throw exceptions constantly through the benchmark, allocating lots of StackTraceElement instances.
• Finally, the cluster is unstable, and some nodes are excluded even though the network seems to be fine and GC pauses are quite small.