Add key_may_exists API

[Congyuwang]

Check if a key may exist without doing any IO.

Notes:

If the key definitely does not exist in the database,
then this method returns False, else True.
If the caller wants to obtain value when the key is found in memory,
fetch should be set to True.
This check is potentially lighter-weight than invoking DB::get().
One way to make this lighter weight is to avoid doing any IOs.

Note that returning True doesn't mean the key definitely exists.
Moreover, if the key does not exist, it might still return True.

The key certainly exists if value is returned.
The key certainly does not exist if False is returned.

Args:

• key: Key to check
• read_opt: ReadOptions

Returns:

• if fetch = False,
  • True if the key may exist.
  • False if the key definitely does not exist.
• if fetch = True,
  • (True, value) if key is found and definitely exist.
  • (False, None) the key definitely does not exist.
  • (True, None) the key may exist.

def key_may_exist(self,
                  key: Union[str, int, float, bytes, bool],
                  fetch: bool = False,
                  read_opt = None) -> Union[bool, Tuple[bool, Any]]: ...