[Phase 1][colocation] Set colocation_id consistently in schemas

[jaki]

Jira Link: [DB-376](https://yugabyte.atlassian.net/browse/DB-376)
pgtable_id (now colocation_id) for schemas are haphazardly set.

• Create table request from postgres does not set it
• CatalogManager::CreateTable does not set it
• RaftGroupMetadata::AddTable does set it
• CatalogManager::AlterTable does set it
• CatalogManager::AddIndexInfoToTable does not set it
• CatalogManager::MarkIndexInfoFromTableForDeletion does not set it
• RaftGroupMetadata::SetSchema does set it (since commit d92b234)

Ideally, pgtable_id would be set from wherever it originates from: postgres create table and alter table requests to master. That way, we don't have to set it ad hoc on master and tserver. However, this may cause backwards compatibility issues.

Another option is to set it from master: CatalogManager::CreateTable and ((CatalogManager::AddIndexInfoToTable and CatalogManager::MarkIndexInfoFromTableForDeletion) or AsyncAlterTable::SendRequest). This enables us to get rid of the ad hoc setting at tserver.

[siddharth2411]

As part of this issue, we essentially want to make sure that colocation_id do not get modified. To do this, we want to put checks on colocation_id wherever it can possibly get changed. There might be some other methods than the ones mentioned above which may change it.

• Verify all the PB in CatalogManager that store colocation_id and ensure none of the function that use these PBs are modifying the colocation_id.
• Verify that colocation_id is being set (correctly) only once in RaftGroupMetadata.

[frozenspider]

Current state of things:

• CatalogManager::CreateTable does set colocation_id - which makes sense since it's an entry point for all table creation.
• RaftGroupMetadata::AddTable does not set it, but it does verify colocation_id remains unchanged
• CatalogManager::AlterTable does not set it - AlterTableRequestPB has no field for that
• CatalogManager::AddIndexInfoToTable does not set it, for the same reasons. Moreover, it's called from CatalogManager::CreateTable
• CatalogManager::MarkIndexInfoFromTableForDeletion does not set it
• RaftGroupMetadata::SetSchema does not set it, but it does verify colocation_id remains unchanged

To summarize, colocation_id is only set in CatalogManager::CreateTable which is consistent. Closing this issue.