Faster way to calculate utf8Length

[djones6]

Description

Applies the same performance fix for utf8 length calculation applied in Kitura/Kitura#1369 to Kitura-net, resolves a performance regression when building with Swift 5.0 snapshots.

Motivation and Context

See Kitura/Kitura#1372

How Has This Been Tested?

The Kitura-net tests pass.
Performance evaluation of a Kitura 'hello-world' app show significant improvement for Swift 5 (and a small improvement for Swift 4.2.1), eliminating the regression seen with Swift 5 builds:

               | Throughput (req/s)      | CPU (%) | Mem (kb)     | Latency (ms)                   | good
Implementation | Average    | Max        | Average | Avg peak RSS | Average  | 99%      | Max      | iters
---------------|------------|------------|---------|--------------|----------|----------|----------|-------
           421 |    63587.4 |    65449.0 |    97.2 |        26145 |      2.0 |      5.4 |    202.8 |    10
       421_fix |    64299.5 |    66892.2 |    97.5 |        26094 |      2.0 |      4.1 |    203.0 |    10
            50 |    52964.2 |    54838.8 |    99.0 |        29153 |      2.4 |      3.4 |    202.0 |    10
        50_fix |    67132.1 |    68740.2 |    97.6 |        29196 |      1.9 |      4.4 |    201.5 |    10

Checklist:

• I have submitted a CLA form
• If applicable, I have updated the documentation accordingly.
• If applicable, I have added tests to cover my changes.

[ianpartridge]

Great. Let's also get rid of this one: https://github.com/IBM-Swift/Kitura-net/blob/321b8b3bd1e809839afa8121bba2760e5ffeb17a/Sources/KituraNet/HTTP/HTTPServer.swift#L495