Allocations are reduced by implementing custom json parser via fastjson
lib.
The change also re-uses `promInstant` object in attempt to reduce number
of
allocations when parsing big responses, as usually happens with heavy
recording rules.
```
name old allocs/op new allocs/op delta
ParsePrometheusResponse/Instant-10 9.65k ± 0% 5.60k ± 0% ~ (p=1.000 n=1+1)
```
Signed-off-by: hagen1778 <roman@victoriametrics.com>
---------
Signed-off-by: hagen1778 <roman@victoriametrics.com>
