Fix IO wait in initExtensionUpdatesFromDynamoDB

[jumde]

goroutine 32 [runnable]:
app/vendor/github.com/brave/go-update/controller.initExtensionUpdatesFromDynamoDB()
        /go/src/app/vendor/github.com/brave/go-update/controller/controller.go:75 +0x3ed
app/vendor/github.com/brave/go-update/controller.RefreshExtensionsTicker.func1(0xc420390000, 0xad5220)
        /go/src/app/vendor/github.com/brave/go-update/controller/controller.go:88 +0x58
created by app/vendor/github.com/brave/go-update/controller.RefreshExtensionsTicker
        /go/src/app/vendor/github.com/brave/go-update/controller/controller.go:86 +0x63
goroutine 52 [IO wait]:
internal/poll.runtime_pollWait(0x7f275505ba20, 0x72, 0xc420735a68)
        /usr/local/go/src/runtime/netpoll.go:173 +0x57
internal/poll.(*pollDesc).wait(0xc420394a98, 0x72, 0xffffffffffffff00, 0xb5b9e0, 0xe0d4d0)
        /usr/local/go/src/internal/poll/fd_poll_runtime.go:85 +0x9b
internal/poll.(*pollDesc).waitRead(0xc420394a98, 0xc4202c9000, 0x1000, 0x1000)
        /usr/local/go/src/internal/poll/fd_poll_runtime.go:90 +0x3d
internal/poll.(*FD).Read(0xc420394a80, 0xc4202c9000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
        /usr/local/go/src/internal/poll/fd_unix.go:157 +0x17d
net.(*netFD).Read(0xc420394a80, 0xc4202c9000, 0x1000, 0x1000, 0xc420735b78, 0x65cf1a, 0xc4205c6638)
        /usr/local/go/src/net/fd_unix.go:202 +0x4f
net.(*conn).Read(0xc4204c6188, 0xc4202c9000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
        /usr/local/go/src/net/net.go:176 +0x6a
net/http.(*connReader).Read(0xc4205c6630, 0xc4202c9000, 0x1000, 0x1000, 0x0, 0x0, 0x0)
        /usr/local/go/src/net/http/server.go:764 +0xf8
bufio.(*Reader).fill(0xc42033c3c0)
        /usr/local/go/src/bufio/bufio.go:100 +0x11e
bufio.(*Reader).Peek(0xc42033c3c0, 0x4, 0x0, 0x0, 0x0, 0x0, 0xc420735ce0)
        /usr/local/go/src/bufio/bufio.go:132 +0x3a
net/http.(*conn).readRequest(0xc4204b8140, 0xb61260, 0xc420060d00, 0x0, 0x0, 0x0)
        /usr/local/go/src/net/http/server.go:941 +0xb5f
net/http.(*conn).serve(0xc4204b8140, 0xb61260, 0xc420060d00)
        /usr/local/go/src/net/http/server.go:1768 +0x4dc
created by net/http.(*Server).Serve
        /usr/local/go/src/net/http/server.go:2798 +0x27b