Update HttpProjectConfigManager to use Last-Modified time

[aliabbasrizvi]

Summary

We now store and use the Last-Modified time when requesting the datafile. In all requests If-Modified-Since header is set and HttpProjectConfigManager creates new project config if there is a HTTP 200 response.

[mikecdavis]

Minor comments. LGTM

[coveralls]

Pull Request Test Coverage Report for Build 1014

• 16 of 22 (72.73%) changed or added relevant lines in 2 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 89.056%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core-httpclient-impl/src/main/java/com/optimizely/ab/OptimizelyHttpClient.java	0	1	0.0%
core-httpclient-impl/src/main/java/com/optimizely/ab/config/HttpProjectConfigManager.java	16	21	76.19%

Files with Coverage Reduction	New Missed Lines	%
core-httpclient-impl/src/main/java/com/optimizely/ab/config/HttpProjectConfigManager.java	7	65.82%

Totals
Change from base Build 1012:	-0.3%
Covered Lines:	3084
Relevant Lines:	3463

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1019

• 16 of 22 (72.73%) changed or added relevant lines in 2 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 89.056%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core-httpclient-impl/src/main/java/com/optimizely/ab/OptimizelyHttpClient.java	0	1	0.0%
core-httpclient-impl/src/main/java/com/optimizely/ab/config/HttpProjectConfigManager.java	16	21	76.19%

Files with Coverage Reduction	New Missed Lines	%
core-httpclient-impl/src/main/java/com/optimizely/ab/config/HttpProjectConfigManager.java	7	65.82%

Totals
Change from base Build 1012:	-0.3%
Covered Lines:	3084
Relevant Lines:	3463

---

💛 - Coveralls