Make ClientBuilder directly accept boxed handler types
#6368
| Job | Run time |
|---|---|
| 47s | |
| 34s | |
| 29s | |
| 25s | |
| 28s | |
| 52s | |
| 2m 47s | |
| 44s | |
| 42s | |
| 2m 22s | |
| 1m 2s | |
| 1m 27s | |
| 1m 7s | |
| 38s | |
| 37s | |
| 1m 0s | |
| 1m 5s | |
| 0s | |
| 57s | |
| 1m 8s | |
| 1m 3s | |
| 1m 1s | |
| 21m 15s |