Faster lookup of Vertx HTTP headers

[franz1981]

In the OpenTelemetry benchmark (run by @barreiro and @brunobat ) I've spotted some memory and cpu waste around VertxUtil related how we lookup for known HTTP headers and how we parse them:

• the former is using a common String literal instead of a constant known AsciiString, which equals/hashCode computations are optimized by Netty
• the latter because we use String::split which create useless intermediate array objects (and Strings) and doesn't make use of faster and intrinsified methods to find the comma/colon delimiters (which instead can be done using String::indexOf)

[quarkus-bot[bot]]

/cc @brunobat (opentelemetry), @radcortez (opentelemetry)

[franz1981]

I'm parking this here till we get some reliable estimation of overhead of the mentioned util methods

[brunobat]

Thanks @franz1981 !