* groupcache: enable H2C + add benchmarks

Add h2c benchmarks. Somehow h2c is slowed than regular HTTP locally for
me. Probably because these aren't real life benchmarks i.e. establishing
TCP connections locally has no cost. And if there is only one operation
happening at any time then h2 has to do more work hence more CPU usage.

With 500 parallel requests going on at the same time, h2 becomes faster
than (or is at the same level as) h1 while using a minimal number of TCP
connections. Thus, it will work much faster in real life situations. So,
let's enable it.

```
name                                                               time/op
GroupcacheRetrieval/h2c/seq-16                                      227µs ± 1%
GroupcacheRetrieval/h2c/parallel=500-16                            54.8µs ±20%
GroupcacheRetrieval/h1,_max_one_TCP_connection/seq-16               150µs ± 2%
GroupcacheRetrieval/h1,_max_one_TCP_connection/parallel=500-16      146µs ± 1%
GroupcacheRetrieval/h1,_unlimited_TCP_connections/seq-16            145µs ± 3%
GroupcacheRetrieval/h1,_unlimited_TCP_connections/parallel=500-16  52.8µs ± 9%

name                                                               alloc/op
GroupcacheRetrieval/h2c/seq-16                                      183kB ± 0%
GroupcacheRetrieval/h2c/parallel=500-16                             143kB ± 1%
GroupcacheRetrieval/h1,_max_one_TCP_connection/seq-16               161kB ± 0%
GroupcacheRetrieval/h1,_max_one_TCP_connection/parallel=500-16      162kB ± 0%
GroupcacheRetrieval/h1,_unlimited_TCP_connections/seq-16            161kB ± 0%
GroupcacheRetrieval/h1,_unlimited_TCP_connections/parallel=500-16   116kB ± 2%

name                                                               allocs/op
GroupcacheRetrieval/h2c/seq-16                                        283 ± 0%
GroupcacheRetrieval/h2c/parallel=500-16                               256 ± 1%
GroupcacheRetrieval/h1,_max_one_TCP_connection/seq-16                 260 ± 0%
GroupcacheRetrieval/h1,_max_one_TCP_connection/parallel=500-16        262 ± 0%
GroupcacheRetrieval/h1,_unlimited_TCP_connections/seq-16              260 ± 0%
GroupcacheRetrieval/h1,_unlimited_TCP_connections/parallel=500-16     279 ± 1%
```

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

* Add fix according to Kemal's suggestion

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>