fix: retry caldav init at startup

vendor/github.com/avast/retry-go/appveyor.yml

@@ -0,0 +1,19 @@
+version: "{build}"
+
+clone_folder: c:\Users\appveyor\go\src\github.com\avast\retry-go
+
+#os: Windows Server 2012 R2
+platform: x64
+
+install:
+  - copy c:\MinGW\bin\mingw32-make.exe c:\MinGW\bin\make.exe
+  - set GOPATH=C:\Users\appveyor\go
+  - set PATH=%PATH%;c:\MinGW\bin
+  - set PATH=%PATH%;%GOPATH%\bin;c:\go\bin
+  - set GOBIN=%GOPATH%\bin
+  - go version
+  - go env
+  - make setup
+
+build_script:
+  - make ci