Lock allocation of IP address

current logic is not safe as it will allow an IP that isnt persisted to
the DB to be given out multiple times if machines joins in quick
succession.

This adds a lock around the "get ip" and machine registration and save
to DB so we ensure thiis isnt happning.

Currently this had to be done three places, which is silly, and outlined
in #294.

app.go

@@ -153,6 +153,8 @@ type Headscale struct {
 	oidcStateCache *cache.Cache
 
 	requestedExpiryCache *cache.Cache
+
+	ipAllocationMutex sync.Mutex
 }
 
 // Look up the TLS constant relative to user-supplied TLS client