[SPARK-17990][SPARK-18302][SQL] correct several partition related behaviours of ExternalCatalog

## What changes were proposed in this pull request?

This PR corrects several partition related behaviors of `ExternalCatalog`:

1. default partition location should not always lower case the partition column names in path string(fix `HiveExternalCatalog`)
2. rename partition should not always lower case the partition column names in updated partition path string(fix `HiveExternalCatalog`)
3. rename partition should update the partition location only for managed table(fix `InMemoryCatalog`)
4. create partition with existing directory should be fine(fix `InMemoryCatalog`)
5. create partition with non-existing directory should create that directory(fix `InMemoryCatalog`)
6. drop partition from external table should not delete the directory(fix `InMemoryCatalog`)

## How was this patch tested?

new tests in `ExternalCatalogSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #15797 from cloud-fan/partition.