ddl.get_schema() must be workable in trigger functions

[no1seman]

According to rule #3: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_txn_management/ we can't use space_object:format() from any kind of transactional triggers, but in some cases, for example wen resolving conflicts we need to get space format. ddl.get_schema() or similar new method must be callable from on_commit, on_rollback, before_replace, on_replace triggers functions.

[Totktonada]

Now the ddl.get_schema() function collects the schema from the space format. So, it seems, this task will be resolved, when we'll implement support of external information schema storage.

[no1seman]

Seems that if we will have all schema data consistently cached - than there will no any limitations to use ddl methods even inside triggers and transactons.