Refine checkpoint logic in MyRocks

[yoshinorim]

I'd like checkpoint logic to be refactored as follows.

1. Rename variable name from rocksdb_snapshot_dir to something like rocksdb_create_checkpoint.
   The command creates checkpoint. Something like "create" "checkpoint" sounds better to me.

2. Return errors to the client if failing to create checkpoint.
   Currently the command does not return any error is failing to create checkpoint. This makes debugging hard.

3. Make it more robust
   Currently 'set global rocksdb_snapshot_dir="/data/mysql/backup/snapshot/"; ' command fails but 'set global rocksdb_snapshot_dir="/data/mysql/backup/snapshot"; ' succeeds. The former fails because "Invalid argument: Directory exists". This is a bit confusing and should accept both.

4. Add more test cases covering the above in rocksdb.checkpoint