Session add/lookup using sync map

[krsna1729]

This PR addresses #62

The number of places session lookup touches is quite a bit. Things i am fuzzy around -

• There is nothing to connect Session and the Conn, so we have to pass TEIDSessionMap OR we can add to Session struct (needed in NewSession)
• TEID being unique per Conn (this PR) vs Conn+IfType (existing impl)
• Need for mutexes around sync.Map
• Test changes made

[krsna1729]

Woops, need to update the examples

[krsna1729]

Do let me know if this is a good approach.

[krsna1729]

Package tests and benchmarks

ok  	github.com/wmnsk/go-gtp/v2	1.009s	coverage: 38.1% of statements
Success: Tests passed.

goos: linux
goarch: amd64
pkg: github.com/wmnsk/go-gtp/v2
BenchmarkAddSessionExist0-88      	   95989	     12159 ns/op	   17240 B/op	      20 allocs/op
BenchmarkAddSessionExist100-88    	   91687	     12796 ns/op	   17240 B/op	      20 allocs/op
BenchmarkAddSessionExist1K-88     	   99243	     10516 ns/op	   17240 B/op	      20 allocs/op
BenchmarkAddSessionExist10K-88    	  116122	      8886 ns/op	   17240 B/op	      20 allocs/op
PASS
ok  	github.com/wmnsk/go-gtp/v2	5.731s
Success: Benchmarks passed.

[wmnsk]

Thank you! Could you check my inline comments?

[wmnsk]

100 times of retry seems insufficient in case of millions of subscribers exist. How about using 0xffff instead of 100 considering the unluckiest situation(only one value remaining unassigned)? Also, logging to let user know it's taking time at some point(e.g., when i = 1000, 10000, ...) can be helpful.

[wmnsk]

I don't think these fields should not be exported as it's no use for manipulating from outside the package(rather, might be dangerous), and the new Session() method satisfies the purpose of removed Session field.

[wmnsk]

Thank you for updating. Now everything looks good to me.