feat: add KeyRangeCache for location-aware routing

This commit adds the KeyRangeCache class that maps key ranges to
specific server locations for routing decisions.

Key features:
- TabletEntry class for tablet metadata (UID, server address, incarnation)
- ServerEntry class for server connection management
- Key range to tablet mapping with efficient lookup
- Lazy server initialization for on-demand connections
- Integration with ChannelFinderServer interfaces

This is part of the experimental location-aware routing for improved latency.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rahul2393