Summary:
New route handle SetDistributionTargetRoute stamps the distributionTargetRegion fiber-local (inside runWithLocals) and delegates to its child, so the ucache pool's auto-wrapped DistributionRoute broadcasts the write to DL. C++ half of moving the ads adm:* global-replication prefixes off legacy fabric broadcast onto DL.

route(): (a) target already stamped (e.g. RootRoute's directed stamp) -> delegate; (b) non-empty routing prefix -> delegate (don't convert directed into broadcast); (c) empty prefix -> stamp target_region ("" = broadcast) and route child in-scope. Lives in routes/ (no FOLLY_ATTR_WEAK), registered in buildRouteMap().

Also bumps mcrouter ClientVersion to 182 so the config can gate DL on mc-client-version >= 182. Land before the configerator change.

Reviewed By: stuclar

Differential Revision: D107156224

fbshipit-source-id: 8db84ba22df57827b897c7b6c77c767f7a9b5de6