Clean up Central's scannedPeripherals dict.

When COVIDSafe removed the timer code, it also removed the reset of this
dictionary, so currently it grows and grows in size. It probably doesn't get
large enough to cause real problems (e.g. the OS killing the app), but for
completeness also reset it on a memory warning. By my scrap paper calculations,
a group of 50 people (with identifiers changing every 15m) will get it to ~200kb
in 2 hours, so potentially over days it could cause issues currently.

CovidSafe/CentralController.swift

@@ -29,6 +29,7 @@ class CentralController: NSObject {
     private var central: CBCentralManager?
     private var recoveredPeripherals: [CBPeripheral] = []
     private var queue: DispatchQueue
+    private var lastCleanedScannedPeripherals = Date()
     
     // This dict is to keep track of discovered android devices, so that i do not connect to the same android device multiple times within the same BluetraceConfig.CentralScanInterval
     private var discoveredAndroidPeriManufacturerToUUIDMap = [Data: UUID]()
@@ -42,6 +43,12 @@ class CentralController: NSObject {
     public init(queue: DispatchQueue) {
         self.queue = queue
         super.init()
+        NotificationCenter.default.addObserver(
+          forName: UIApplication.didReceiveMemoryWarningNotification,
+          object: nil,
+          queue: .main) { [weak self] notification in
+            self?.cleanupScannedPeripherals()
+        }
     }
     
     func turnOn() {
@@ -94,6 +101,14 @@ class CentralController: NSObject {
         return false
     }
     
+    fileprivate func cleanupScannedPeripherals() {
+        scannedPeripherals = scannedPeripherals.filter { scan in
+            guard let encounterTime = scan.value.encounter.timestamp else { return true }
+            return abs(encounterTime.timeIntervalSinceNow) < BluetraceConfig.CentralScanInterval
+        }
+        lastCleanedScannedPeripherals = Date()
+    }
+    
     public func getState() -> CBManagerState? {
         return central?.state
     }
@@ -181,6 +196,9 @@ extension CentralController: CBCentralManagerDelegate {
                          "advertisments": advertisementData as AnyObject] as AnyObject
         
         DLog("\(debugLogs)")
+        if (abs(lastCleanedScannedPeripherals.timeIntervalSince(Date())) > BluetraceConfig.CentralScanInterval) {
+            cleanupScannedPeripherals()
+        }
         var initialEncounter = EncounterRecord(rssi: RSSI.doubleValue, txPower: advertisementData[CBAdvertisementDataTxPowerLevelKey] as? Double)
         initialEncounter.timestamp = nil