Adding 6 methods on 'exported', 'logger' and 'entry':
- IsDebugEnabled() bool
- IsInfoEnabled() bool
- IsWarnEnabled() bool
- IsErrorEnabled() bool
- IsFatalEnabled() bool
- IsPanicEnabled() bool
Replace duplicated 'if logger.level() >= XxxxLevel' by a call to the new methods in 'logger' and 'entry'
Closes#761
`std.Level` is protected by mutex in setter (SetLevel),
so it must be protected in geetter (GetLevel) too.
Signed-off-by: Anton Tiurin <noxiouz@yandex.ru>