feat: Add cache for NetService state

With previous implementation, NetService would do reachability tests
too often and sometimes multiple times in parallel (as we can call
`isOnline` from multiple parallel `client.query()` calls)

This is problematic as it can unnecessary slow down the app processes

This commit caches the isOnline result and register to the `NetInfo`
event listener so stay up-to-date, so we can drastically reduce
reachability tests on Cozy's servers

src/pouchdb/platformReactNative.isOnline.ts

@@ -1,5 +1,15 @@
+import NetInfo from '@react-native-community/netinfo'
+
 import { NetService } from '/libs/services/NetService'
 
+let currentState: boolean | undefined = undefined
+
 export const isOnline = async (): Promise<boolean> => {
-  return (await NetService.isConnected()) ?? true
+  if (currentState === undefined) {
+    currentState = (await NetService.isConnected()) ?? true
+    NetInfo.addEventListener(state => {
+      currentState = state.isConnected ?? true
+    })
+  }
+  return currentState
 }