Significant changes:
1. `NewChecker()` accepts no argument, `zeroLinger` is enabled by default
2. Optional parameter `zeroLinger` in `CheckAddr()` turned into `CheckerAddrZeroLinger()`
3. `InitChecker()` -> `CheckingLoop(context)`
4. `ErrNotInitialized` -> `ErrCheckerAlreadyStarted`
5. Fewer system call calling, e.g. `connect()` is called exactly once per check
6. Epoll is now edge-triggered
1. 'shaker' renamed to 'check', should have named the repo to
tcp-checker
2. Add NewChecker() for creation
3. Add parameter zeroLinger
4. More error handling (Error from syscall.Close is returned)
Tevin Zhang