Return request data to the pool in Ingester.Push()

[bboreham]

After #1526 the unmarshall code is trying to get memory out of the pool, but in ingesters it is never returned so the pool.Get() call just slows things down.

After making this change it still wasn't that great, because the pool was boxing TimeSeries objects. So change those to *TimeSeries.

before:

BenchmarkIngesterPush-2   	     100	  23525939 ns/op	 3881766 B/op	   42317 allocs/op

after:

BenchmarkIngesterPush-2   	     100	  22937648 ns/op	 3273382 B/op	   32482 allocs/op

(note out of the 3-4MB per iteration, 1MB is overhead from the FingerprintLocker)

[codesome]

Pool is better used with pointers 👍. And I assume the BenchmarkIngesterPush code was consistent for the before and after comparison? (Because client.ToWriteRequest is additional in the new test)

[bboreham]

I'm not so sure now what those benchmarks were run against. Here's a new set with more permutations, on a different machine (go version go1.12 linux/amd64):

Current master 7447f75e9a97072e0b7b07d51174e55974d1baa4:
BenchmarkIngesterPush 	     100	  19752857 ns/op	 3280511 B/op	   31703 allocs/op
Before this PR db09d07e546e9c558154cfcf7d6766e06b0466cf: 
BenchmarkIngesterPush 	     100	  21412348 ns/op	 3291262 B/op	   31719 allocs/op
After changing benchmark f35f374c84991f7501da80ff550209f8f5118a9a:
BenchmarkIngesterPush 	      50	  33123151 ns/op	12155937 B/op	   62149 allocs/op
This PR fab5ced5e3c071a930966781b6eb786d8d34cc83:
BenchmarkIngesterPush 	     100	  26417682 ns/op	 3178133 B/op	   32073 allocs/op
Rebased against master d886f6e85a0356e4abbfd76cb685fddb69a8b626:
BenchmarkIngesterPush 	     100	  24700828 ns/op	 3174894 B/op	   32078 allocs/op

I'll merge since it's approved and check we get the expected drop in allocations in my staging environment.