gracefully close connection fixes

[tebruno99]

Original PR: gorilla#487 by @sanketplus

Fixes: gorilla#448 #20

Creating PR to request and accommodate comments. There are multiple areas which requires decision making.>

whether to maintain backwards compatibility with variadic args, or is there a better approach.
after sending a close frame, what to do exactly. Wait for sometime and give up? How long to wait?
Do we need to set kind of read lock for public read methods once close has been initiated?
How exactly do I interpret output of wstest?
<- suggestions