[DynamicPartition] Dynamic partition range always intersect with existing partition's.

[morningman]

Describe the bug
In our cluster, we found a lot warning log in Master fe.log

2020-05-25 14:33:06,763 WARN 44 [DynamicPartitionScheduler.recordCreatePartitionFailedMsg():324] dynamic add partition failed: errCode = 2, detailMessage = Range [types: [DATE]; keys: [2021-03-23]; ‥types: [DATE]; keys: [2021-04-23]; ) is intersected with range: [types: [DATE]; keys: [2021-03-25]; ‥types: [DATE]; keys: [2021-04-25]; ), db: default_cluster:db1, table: palo_tbl

To Reproduce

1. Create table with dynamic partition enabled, and time unit is month.
2. Suppose today is 2020-05-25, it will create partition with range [05-25, 06-25), ...
3. Tomorrow, it will begin to create partition with range [05-26, 06-26)

Expected behavior
We expected to have range [05-01, 06-01), [06-01, 07-01)