Add delay between batches & one more "concurrent map write" fix

[ivanilves]

I try to avoid non-single scope PR but ...

What do we bring?

• -w, --wait-between CLI opt to wait between batches of requests (extra layer of self-DoS protection)
• fix another "concurrent map writes" issue (not funny anymore, raising importance of Sporadic CI concurrent map write failures #143) goroutine fatal when pushing big amount of images #154
• move registry package from internal to v1 namespace (a stupid chore to make me happy)

GIF

[coveralls]

Coverage increased (+0.01%) to 97.291% when pulling 85a2485 on batch-delay into 9f60bca on master.

[vonrabbe]

PR title is not true, but OK, the PR itself LGTM

[vonrabbe]

I've changed the PR title BTW! We need to be more accurate in our descriptions 😉