Optimizations for GenerateClientUrl

These optimizations bring the inclusive time of GenerateClientUrl (as a
percentage of total) from 3.15%-2.01% for the Plan9_Gallery perf test

 - Branch on number of path-parts to combine in BuildUrl to favor
   string.concat where it makes sense and avoid instantiating
   StringBuilders that aren't used.

 - Perform a single UrlPathEncode operation (instead of N operations)

 - Remove Cookieless-Session support for WebPages urls, this was never
   supported for WebPages, and introduced a regression of .5% of total
   execution time for this test.