datastore: add clear_table and fix drop_table

[Centril]

Description of Changes

Aternative to and closes #3210.
This version relies on pending_schema_changes.
The first commit adds clear_table to the datastore that's efficient and can be exposed to the module ABI in a follow up.
The second commit fixes drop_table.

API and ABI breaking changes

None

Expected complexity level and risk

3?

Testing

test_drop_table_is_transactional is amended to check TxData.

[Shubham8287]

Looks Good, thanks!
Considering there will be a follow-up PR to fix this for commitlog replay.
This PR will make commitlog replay logic to empty dropped tables but we still need a patch to actually drop them from memory.

[gefjon]

Holding off on approving until we get the replay parts of this merged and tested, but I like this.

[Centril]

Holding off on approving until we get the replay parts of this merged and tested, but I like this.

I've fixed the replay parts, but we cannot integration test this until the add-column PR as simply dropping a table is not allowed.