This change introduces following simplified steps to follow
during config migration.
```
// Steps to move from version N to version N+1
// 1. Add new struct serverConfigVN+1 in config-versions.go
// 2. Set configCurrentVersion to "N+1"
// 3. Set serverConfigCurrent to serverConfigVN+1
// 4. Add new migration function (ex. func migrateVNToVN+1()) in config-migrate.go
// 5. Call migrateVNToVN+1() from migrateConfig() in config-migrate.go
// 6. Make changes in config-current_test.go for any test change
```
url.Parse() wrongly parses an address of format "address:port"
which is fixed in go1.8. This inculcates a breaking change
on our end. We should fix this wrong usage everywhere so that
migrating to go1.8 eventually becomes smoother.
checkURL() is a generic function to check if a passed address
is valid. This commit adds support for addresses like `m1`
and `172.16.3.1` which is needed in MySQL and NATS. This commit
also adds tests.
This is an attempt cleanup code and keep the top level config
functions simpler and easy to understand where as move the
notifier related code and logger setter/getter methods as part
of their own struct.
Locks are now held properly not globally by configMutex, but
instead as private variables.
Final fix for #3700
