Support React 16

Use refs by callback instead of strings key.

Expose directionalLockEnabled attribute

Crash on iOS 11 during RCTWKWebViewManager destruction

I basically let RCTWKWebViewManager stop holding a reference to WKProcessPool. Just left the pool bound to the
WKWebView.

Renamed _SwizzleHelper to _SwizzleHelperWK

This can help to avoid compile symbol conflict when using react-native-webview-bridge

Add canGoBack and canGoForward methods

Add prop to remove keyboard accessory view

Use shared process pool to share cookies between instances

# Conflicts:
#	ios/RCTWKWebView.xcodeproj/project.xcworkspace/xcuserdata/insraq.xcuserdatad/UserInterfaceState.xcuserstate

Fix retain cycle