feat: randomly allocate a isolation level for txn when create test case

[gukj-spel]

pr for issue #42, randomly allocate a isolation level for every txn when create test case

• add isolation level message in description
• isolation level: READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ,SERIALIZABLE
• for example: II0-ICI1-ICI2.txt , the description is follow

  #
  # Test case description
  # POPG Pattern: T1 ==II0==> T2 ==ICI1==> T3 ==ICI2==> T1
  # Parameters: #column=2 #txn=3 #operations=3 #variable=3
  # Structure: Sequence-Session-Query
  # Txn Isolation: T1:REPEATABLE-READ,T2:READ-COMMITTED,T3:REPEATABLE-READ
  # When sequence=0, it is a preparation phase, otherwise an execution phase
  #
  

[hldgs]

why print this "description += "# When sequence=0, it is a preparation phase, otherwise an execution phase" + "\n"" 2 times ?

[gukj-spel]

Oh! my mistake, I'll fix it.

[hldgs]

these 4 isolation level is just for mysql or some similar dbms, other dbms such as mongodb, oceanbase doesn't support the same isolation level

[hldgs]

we can support your feature in mysql, and then extend it to other dbms. it's essential to limit the scope of your current commit to mysql, do you have any idea?

[gukj-spel]

Ok, I'll limit the scope of the isolation level. Since the first parameter of mda_generate is dbms type, my idea is to define different isolation level for different dbms. I will write a function to do this.

[hldgs]

ok, and please make sure your commit can be run on every dbms you want to support. I suggest you firstly add a function to control the scope, and limit it on mysql. And maybe we can extend to other dbms one by one