Implement a very lazy translation of headers

[pushkarnk]

This pull request proposes a very lazy translation of HTTPHeaders to HeadersContainer. The difference between these structures, that matters most, is the conformance to Collection. HeadersContainer is a Collection, HTTPHeaders isn't.

Conforming HTTPHeaders to Collection via an extension isn't straightforward. A conformance to Collection using the existing API leads to a very inelegant and sub-optimal implementation.

In that case, the only alternative we are left with is an "as lazy as possible" translation. Inside HeadersContainer, we maintain an extra backing store of type HTTPHeaders. The existing backing store, a Dictionary lies dormant. Only when the user code tries to access the HeadersContainer as Collection, a translation is done and the Dictionary backing store comes to life. Subsequently, we update both the backing stores. This is to make sure there's no need for the HeadersContainer to HTTPHeaders translation.

In short, the HTTPHeaders -> HeadersContainer is done very lazily and the vice versa isn't needed.

[pushkarnk]

On TechEmpower/JSON this yields a benefit of around 8-9%.

[pushkarnk]

Also, I've realised that though the Kitura tests have enough test coverage for HeadersContainer, the Kitura-NIO repo is negligible coverage. I'm planning to write tests as a part of this pull request.

[pushkarnk]

Looks like a change to BlueSSLService is breaking Kitura-NIO on Darwin

Kitura/BlueSSLService@468d581#diff-92ba0ed683849bc916d3ff460ce0b4b6

[nethraravindran]

BlueSSLService bug has been fixed in Kitura/BlueSSLService#67 and tagged

[pushkarnk]

A summary of the perf gains this PR gives on the different TechEmpower benchmarks (column lazy indicates the new numbers). All measurements are in MB/s.

Plaintext(Number of connections)	1.0.7	lazy	KituraNet
256	12.02	14.5	14.83
1024	11.93	14.17	13.66
4096	11.56	14.16	12.21
16384	11.57	13.73	11.37

JSON (Number of connections)	1.0.7	lazy	KituraNet
8	5.43	6.67	8.81
16	7.79	8.89	11.29
32	9.26	10.73	11.39
64	9.39	11.4	11.37
128	9.42	11.6	11.41
256	9.49	11.66	11.4

DB(Number of connections)	1.0.7	lazy	KituraNet
8	2.96	3.18	3.59
16	4.39	4.79	5.21
32	4.82	5.24	6.04
64	4.87	5.41	6.61
128	4.9	5.56	6.81
256	5.07	5.64	6.82
Updates(Number of queries)	1.0.7	lazy	KituraNet
--	--	--	--
1	2.96	3.1	5.16
5	1.71	1.71	1.8
10	1.39	1.4	1.43
15	1.27	1.27	1.29
25	1.16	1.16	1.18

Queries (Number of queries)	1.07	lazy	KituraNet
1	3.94	4.18	5.12
5	2.63	2.71	3.02
10	2.41	2.45	2.58
15	2.29	2.31	2.4
20	2.24	2.23	2.3
25	2.2	2.17	2.25

[pushkarnk]

For the records, these are the %age gains on the different TechEmpower benchmarks:

Plaintext: 18-22%
JSON: 14-23%
DB: 7-13%
Updates: 0-4%
Queries: 0-6%