Extract replication_strategy_type enum from locator/abstract_replication_strategy.hh
into a new lightweight header locator/replication_strategy_type.hh, and use it in
db/config.hh instead of the full abstract_replication_strategy.hh.
abstract_replication_strategy.hh pulls in a large transitive dependency tree
(schema.hh, mutation serializers, etc.) costing ~1.7s per file. With this change,
config.hh's incremental parse cost drops from 1.7s to 0.6s. Since ~85 files
include config.hh without also including database.hh (which would bring in these
deps anyway), this saves ~93s total CPU.
Speedup: part of a series measured at -5.8% wall-clock improvement
(same-session A/B: 16m14s -> 15m17s at -j16, 16 cores).
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