Allow nesting Query Analyzers

In some circumstances we should allow to nest Query Analyzers. However, we should only initialize new ones within the block.

The usage pattern for which it is needed:

1. We use default analyzers in regular execution (spec/support/database/query_analyzer.rb)
2. We register a single purpose analyzer within a block of execution (testing of background migrations via lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb)

Related #358430 (closed)