fix(encoding): Improve array string encoding perf a little more
fix(encoding): Improve array string encoding perf a little more

Encode the compressed data at the start internal buffer. This ensures
the returned slice maintains the entire capacity and is available for
subsequent use.

When we pool / reuse string buffers, this will help considerably.

Improvements over previous commit:

```
name                        old time/op    new time/op    delta
EncodeStrings/10/batch-8       542ns ± 1%     355ns ± 2%   -34.53%  (p=0.008 n=5+5)
EncodeStrings/100/batch-8     5.29µs ± 1%    3.58µs ± 2%   -32.20%  (p=0.008 n=5+5)
EncodeStrings/1000/batch-8    48.6µs ± 0%    36.2µs ± 2%   -25.40%  (p=0.008 n=5+5)

name                        old alloc/op   new alloc/op   delta
EncodeStrings/10/batch-8        704B ± 0%        0B       -100.00%  (p=0.008 n=5+5)
EncodeStrings/100/batch-8     9.47kB ± 0%    0.00kB       -100.00%  (p=0.008 n=5+5)
EncodeStrings/1000/batch-8    90.1kB ± 0%     0.0kB       -100.00%  (p=0.008 n=5+5)

name                        old allocs/op  new allocs/op  delta
EncodeStrings/10/batch-8        0.00           0.00           ~     (all equal)
EncodeStrings/100/batch-8       1.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
EncodeStrings/1000/batch-8      1.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)
```

nooproblem/influxdb,li-ang/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,influxdata/influxdb,influxdb/influxdb,influxdb/influxdb,influxdb/influxdb,nooproblem/influxdb,li-ang/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,influxdata/influxdb,influxdb/influxdb,nooproblem/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb