Add an Understanding TLS chapter#4

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mattcaswell wants to merge 7 commits intoopenssl:masterfrom

mattcaswell:understand-tls

Commits on Dec 13, 2017

Add an Understanding TLS chapter
mattcaswell
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Commits on Jan 21, 2018

Make some updates for TLS 1.3
mattcaswell
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Commits on Jan 25, 2018

Some tweaks based on review feedback from Paul Yang
mattcaswell
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Commits on Jan 29, 2018

Fix a Makefile bug
mattcaswell
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Commits on Jan 30, 2018

Add some content on sessions and key updates
mattcaswell
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Commits on Jan 31, 2018

Add some text about renegotiation
mattcaswell
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Commits on Feb 1, 2018

Add a time indicator to the handshake graphic
mattcaswell
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