* initial dns resolver impl
* add srv lookup
* more srv
* change from string to Update
* added port handling
* a complete draft for dns resovler except the polling mechanism
* added sleep to infrequently poll the DNS server
* commented out test case since they are not portable
* remove duplicate declaration/definition in grpclb
* change namespace for grpclb structures
* goimports gofmt
* fix sorting issue
* implement sort.Interface
* different sort implementation for different go version
* add missing files
* fix missing comments
* handle err
* fix comments: unexport dnsResolver and dnsWatcher, add chan to exit Next(), add freq to control polling frequency
* enhance target string handling, add static IPWatcher, add mock test
* debug travis
* disable real address resolver test, since travis return 3 resolved IPs, 2 of them are duplicates
* shorten test time by reduce sleep time in TestIPWatcher, since it doesn't really do the DNS round trip
* resolve data race
* resolve data race using waitgroup
* reimplement setHostPort, compileUpdate, unexport ipWatcher
* delete sort related stuff, fix close bug for Next(), fix compileUpdate bug(change to map[Update]bool, plus fix minor review comments
* fix minor test case
* minor change to Next()
* use net.DefaultResolver with context as input
* add different build rules for lookupHost and lookupSRV
* minor fix
* go1.6 shall fail, but 1.7, 1.8 should pass
* go1.6 is expected to pass
* all go version should pass, added pre17 and 17 for replaceNetFunc to handle context problem
* rename test helper file name. should fix build problem
* goimports
* fix 1.7 context problem
* reformat dns_resolver_test structures
* change Next() behavior to have equal stalling interval between each lookup. Restructure dns_resolver_test.
* gofmt
* update go17_test.go, go18_test.go to reuse code. dns_resolver_test: check result and behavior correctness separately.
* update Next() logic
Menghan Li
lyuxuan