feat: support passing context

[mingrammer]

Sometimes, we can run an operation that takes longer than the timeout of the incoming request. In this case, the whole request should be canceled along with all subtasks including the cache operations. But current implementation doesn't support passing context, so we can't cancel the cache operations even if the current context is expired.

So I added the passing context feature for all cache operations with this PR. It keeps the backward compatibility, thus does not affect existing code using gomemcache.

import (
    "github.com/bradfitz/gomemcache/memcache"
)
func main() {
    mc := memcache.New("10.0.0.1:11211", "10.0.0.2:11211", "10.0.0.3:11212")
    mc.Set(&memcache.Item{Key: "foo", Value: []byte("my value")})
    it, err := mc.Get("foo")

    // With context
    ctx, cancel := context.WithTimeout(2 * time.Second)
    doLongTimeJob(ctx)
    ...
    it, err = mc.GetWithContext(ctx, "bar")
    ...
}

I would appreciate it if you think about this feature in a good way.