Stop sharing lock between write and read

[nakabonne]

All data points are immutable, and all write operation is append-only. Logically any read operation and write operation don't conflict at all.

In spite of that, we currently lock with Mutex whenever reading and writing.

tstorage/memory_partition.go

Lines 178 to 187 in b1bbbaf

	// TODO: Consider to stop using mutex every time.
	// Instead, fix the capacity of points slice, kind of like:
	/*
	m.points := make([]*DataPoint, 1000)
	for i := 0; i < 1000; i++ {
	m.points[i] = point
	}
	*/
	m.mu.Lock()
	defer m.mu.Unlock()

[nakabonne]

Like MVCC (Multi Version Concurrency Control), it's enough to keep a read-only snapshot at a certain point and give back it when reading out.