Use String.split() instead of NSString.components()

[ianpartridge]

Description

Remove the usage of NSString.components() and replace it with String.split().

Motivation and Context

In Swift 5, bridging from String to NSString involves a conversion from UTF-8 to UTF-16. So we want to avoid NSString as much as possible.

How Has This Been Tested?

Ran Kitura-net test suite.

[djones6]

Performance evaluation on Ubuntu 16.04 with a simple query params echo benchmark:

               | Throughput (req/s)      | CPU (%) | Mem (kb)     | Latency (ms)                   | good
Implementation | Average    | Max        | Average | Avg peak RSS | Average  | 99%      | Max      | iters
---------------|------------|------------|---------|--------------|----------|----------|----------|-------
      base_421 |    30848.1 |    31443.5 |    98.2 |        25354 |      4.1 |      4.8 |    201.6 |    10
     split_421 |    31812.4 |    32536.3 |    98.5 |        25715 |      4.0 |      4.8 |    202.6 |    10
       base_50 |    29572.5 |    30033.1 |    98.7 |        26017 |      4.3 |      5.0 |    204.3 |    10
      split_50 |    30675.2 |    31403.9 |    98.3 |        26116 |      4.1 |      4.8 |     50.2 |    10

There's a ~3% improvement to the overall performance with this change. Swift 5 came out behind in this workload however, which implies there may be other NSString usage in the query parameters codepaths to eliminate.