Preprocess `:path` header to avoid repeatedly splitting the header for route picking.

Route matching is performed on each route in a virtual host (linearly until the matched route is found) and each routing matching will perform path matching once. When path matching is performed, it will try to split `:path` header into front part and back part (query param and/or fragment)](https://github.com/envoyproxy/envoy/blob/266c9fa51b0d1dbce746abda6c1b47b3ca19c3cb/source/common/common/matchers.cc#L126). This means splitting is currently done **up to N times** where N is number of routes in a virtual host.

Can we do the splitting instead when the `:path` header is parsed into RequestHeaderMap to avoid repeatedly splitting done during route picking?