Remove ODB transaction nesting

Object database transactions are currently defined in the bulk-checkin subsystem. A transaction is started with begin_odb_transaction() and ended with end_odb_transaction(). Transactions can be nested though. This means begin_odb_transaction() can be invoked multiple times before invoking end_odb_transaction(). In such cases, a transaction only truly commits when the outer most transaction is ended.

This can be rather confusing as it is difficult to know when a transaction is actually flushed and persisted. The only cases where transaction can become nested are in cache-tree.c:cache_tree_update() and read-cache.c:add_files_to_cache(). All other transaction are managed directly by the builtins. This nested transaction mechanism exists to allow for only a single transaction to span across an operation by ensuring nested transactions simply reuse the current transaction.

Instead, we should just avoid creating a transaction if one already exists. This ensures that invoking end_odb_transaction() always commits the transaction.