Transaction support in databend-meta

Databend-meta support transaction with KVApi, transaction parameters are:

• condition: condition is a list of predicates. If all of them succeed, the if_then will be executed, otherwise else_then op will be executed.
• if_then: if_then is a list of operations that will be executed when all condition evaluates to be true.
• else_then: else_then is a list of operations that will be executed when not all condition evaluates to be true.

The condition is a list of TxnCondition, which includes these fields:

• key: String format key to compare with.
• target: The target of key value to be compared with, can be the last success upsert operation sequence number of the key, or the bytes value of the key.
• expected: The expected result of condition, the result includes EQ, GT, etc.

The is_then and else_then is a list of TxnOp, which is one of these types:

• TxnGetRequest get: Get the value and sequence of a key.
• TxnPutRequest put: Put the value of a key.
• TxnDeleteRequest delete: Delete a key.
• TxnDeleteByPrefixRequest delete_by_prefix: Delete all the keys with the prefix string.