When using multiple benchmarks earlier ones affect the ones coming later

I have the following benchmarks in a group:

```haskell
bgroup "map"
[ bench "machines" $ whnf drainM (M.mapping (+1))
, bench "streaming" $ whnf drainS (S.map (+1))
, bench "pipes" $ whnf drainP (P.map (+1))
, bench "conduit" $ whnf drainC (C.map (+1))
, bench "list-transformer" $ whnf drainL (lift . return . (+1))
]
```

The last two benchmarks take significantly more time when I run all these benchmarks in one go using ``stack bench --benchmark-arguments "-m glob ops/map/*"``.

```
$ stack bench --benchmark-arguments "-m glob ops/map/*"

benchmarking ops/map/machines
time 30.23 ms (29.22 ms .. 31.04 ms)

benchmarking ops/map/streaming
time 17.91 ms (17.48 ms .. 18.37 ms)

benchmarking ops/map/pipes
time 29.30 ms (28.12 ms .. 30.03 ms)

benchmarking ops/map/conduit
time 36.69 ms (35.73 ms .. 37.58 ms)

benchmarking ops/map/list-transformer
time 84.06 ms (75.02 ms .. 90.34 ms)
```

However when I run individual benchmarks the results are different:

```
$ stack bench --benchmark-arguments "-m glob ops/map/conduit"

benchmarking ops/map/conduit
time 31.64 ms (31.30 ms .. 31.86 ms)

$ stack bench --benchmark-arguments "-m glob ops/map/list-transformer"

benchmarking ops/map/list-transformer
time 68.67 ms (66.84 ms .. 70.96 ms)
```
To reproduce the issue just run those commands in [this repo](https://github.com/harendra-kumar/streaming-benchmarks/tree/a5a329fd10550366de33e53aeb7417c2ae608a8e).

I cannot figure what the problem is here. I tried using "env" to run the benchmarks and putting a "threadDelay" for a few seconds and a "performGC" in it but nothing helps.

I am now resorting to always running each benchmark individually in a separate process. Maybe we can have support for running each benchmark in a separate process in criterion itself to guarantee isolation of benchmarks, as I have seen this sort of problem too often. Now I am always skeptical of the results produced by criterion.