Deprecate AbstractSchemaManager::createSchema() in favor of introspectSchema()

[morozov]

Q	A
Type	deprecation

The current API has the following problems:

1. The createSchema() name is misleading since unlike other create*() methods (e.g. createTable()) which create an object in the database, createSchema() creates an internal representation of the schema by introspecting it.
2. The fact that this method existed didn't allow to introduce a method that would actually create the schema in Replace schema SQL collectors with SQL builders #5416.

This pull request proposes to deprecate AbstractSchemaManager::createSchema() in favor of introspectSchema().

The name of the introspectSchema() method being introduced is slightly inconsistent with AbstractSchemaManager::getTable() recently introduced in #5595. However, instead of naming it getSchema() for consistency, I believe it's not too late to rename getTable() to introspectTable(). That would be more precise.