Merge V5 to master branch

[aldas]

See #2861

If you are using Linux you can migrate easier parts like that:

find . -type f -name "*.go" -exec sed -i 's/ echo.Context/ *echo.Context/g' {} +
find . -type f -name "*.go" -exec sed -i 's/echo\/v4/echo\/v5/g' {} +

or in your favorite IDE

Replace all:

1. echo.Context -> *echo.Context
2. echo/v4 -> echo/v5