Add Integration Macros section explaining the relationship between
CMake options (BUILD_WITH_*) and source code macros (KCENON_WITH_*).

Refs: #23

Define both legacy BUILD_WITH_* macros and new WITH_* macros in
CMake compile definitions to maintain backward compatibility while
enabling KCENON_WITH_* feature flags from feature_flags.h.

Refs: #23

Change forward declarations from legacy namespace aliases to actual class types
to prevent conflicts when network_system/core/messaging_server.h is included.

- Forward declare kcenon::network::core::messaging_server and messaging_session
- Add using aliases for backward compatibility with network_system namespace

Add connection_types.h with server-side implementations of connection pooling
types that were removed from database_system as part of Phase 4.3
(Client Library Diet initiative).

Types added:
- connection_pool_config: Pool configuration parameters
- connection_stats: Statistics for monitoring
- connection_wrapper: Connection metadata wrapper
- connection_pool_base: Abstract base class for pools
- connection_pool: Basic pool implementation

This allows database_server to maintain server-side connection pooling
while database_system clients use ProxyMode for production deployments.

- Document connection_types.h and its contents
- Add note about database_system Phase 4.3 Client Library Diet
- Clarify that connection pooling is now server-side only

Remove redundant BUILD_WITH_* compile definitions from CMakeLists.txt.
The feature_flags.h header from common_system now properly maps WITH_*
symbols to KCENON_WITH_* macros, making the legacy definitions unnecessary.

Changes:
- Replace BUILD_WITH_COMMON_SYSTEM WITH_COMMON_SYSTEM with just WITH_COMMON_SYSTEM
- Replace BUILD_WITH_MONITORING_SYSTEM WITH_MONITORING_SYSTEM with just WITH_MONITORING_SYSTEM
- Replace BUILD_WITH_CONTAINER_SYSTEM WITH_CONTAINER_SYSTEM with just WITH_CONTAINER_SYSTEM
- Update comments to clarify the WITH_* to KCENON_WITH_* mapping

This completes the integration flag normalization for PR #24.

Update Integration Macros section to clearly document the three-layer
mapping:
1. CMake Option (BUILD_WITH_*) - User-facing configuration
2. Preprocessor Symbol (WITH_*) - Compile definition set by CMake
3. Macro (KCENON_WITH_*) - Used in source code via feature_flags.h

This clarifies how BUILD_WITH_* options result in KCENON_WITH_* macros
through the WITH_* intermediate symbols.