Corrections, enhancements to core database component skills

[connormcd]

Bring up to date some of the content in the various skills to reflect best practices and changed functionality.

skills/admin/user-management.md

• added noauth accounts

skills/appdev/json-in-oracle.md

• blob preference plus OSON

skills/appdev/locking-concurrency.md

• corrections to skip locked, ora_rowscn, plus expanded commentary on DDL / locking

skills/appdev/sequences-identity.md

• clarified CACHE usage and added automatic 19c cache management

skills/appdev/transaction-management.md

• corrections to PLSQL commit/rollback handling

skills/architecture/inmemory-column-store.md

• added 16G free

skills/design/partitioning-strategy.md

• brought global indexes up to date with asynch maintenance

skills/design/tablespace-design.md

• remove obsolete advice on index separation, clarified uniform/autoallocate

skills/performance/index-strategy.md

• corrected some bitmap and foreign key index topics

skills/performance/memory-tuning.md

• clarified restrictions on AMM

skills/performance/optimizer-stats.md

• increased emphasis on using defaults