Lock set when in use in `uniqueValues`
Lock set when in use in `uniqueValues`

This change causes the set that `uniqueValues` makes use of to wait when checking until no values are being inserted and to wait when inserting values.
tzongw/ReactiveCocoa,smilypeda/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,calebd/ReactiveCocoa,ikesyo/ReactiveCocoa,tzongw/ReactiveCocoa,ikesyo/ReactiveCocoa,andersio/ReactiveCocoa,andersio/ReactiveCocoa,ohwutup/ReactiveCocoa,ohwutup/ReactiveCocoa,sandyway/ReactiveCocoa,victorlin/ReactiveCocoa,smilypeda/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,victorlin/ReactiveCocoa,calebd/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,walkingsmarts/ReactiveCocoa,rpowelll/ReactiveCocoa,sandyway/ReactiveCocoa,walkingsmarts/ReactiveCocoa,rpowelll/ReactiveCocoa,smilypeda/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ohwutup/ReactiveCocoa,tzongw/ReactiveCocoa,rpowelll/ReactiveCocoa,andersio/ReactiveCocoa,walkingsmarts/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,calebd/ReactiveCocoa,victorlin/ReactiveCocoa,sandyway/ReactiveCocoa