* Unify the names of methods in `TabletManager` which do not require locks
Currently, there are several naming patterns in `TabletManager` class
for methods (mainly private methods) that needs to be executed inside the lock:
1. **`xxx_with_no_lock()`**:
The "with_no_lock" suffix has two meanings: one is not needed,
and the other is that a lock has been added externally;
2. **`xxx_unlock()`**:
"unlock" is a verb and may be mistaken for the need to unlock
a mutex in this method.
3. **`xxx_unlocked()`**:
Note that "unlocked" is an adjective that means the operation
in this method is not locked.
4. **`xxx_locked()`**:
"locked" is also an adjective, meaning that the method is locked.
This is also more likely to be misunderstood: one is already
locked externally; the other is locked internally by the method.
Actually what we really want is `xxx_already_locked`, but this way
the name is a little longer.
5. There is no identification in the method name:
the reader cannot intuitively know whether the method needs to be locked
This patch unifies all the above pattern to be `xxx_unlocked()`, and adjust
some indentation in code style.
Additionally, this patch also remove an unused `add_tablet()` method, because
a new version has already been used.
This patch doesn't contain any functional modifications.
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