Improvements to batch float encoder
Improvements to batch float encoder

- Inlined the closure to avoid a function call.
- Changed append(b, make([]byte, 8)...) to inline the make call.
- Check for NaN once at the end assuming NaN is infrequent.

New performance delta comparing the current iterators to the new batch
function:

name                   old time/op    new time/op    delta
EncodeFloats/10_seq      1.32µs ± 2%    0.17µs ± 2%  -87.39%  (p=0.000 n=10+10)
EncodeFloats/10_ran      2.09µs ± 1%    0.15µs ± 0%  -92.97%  (p=0.000 n=10+9)
EncodeFloats/100_seq     8.37µs ± 2%    1.28µs ± 2%  -84.74%  (p=0.000 n=10+10)
EncodeFloats/100_ran     19.1µs ± 1%     1.3µs ± 1%  -93.08%  (p=0.000 n=9+9)
EncodeFloats/1000_seq    60.4µs ± 1%    12.6µs ± 0%  -79.13%  (p=0.000 n=9+7)
EncodeFloats/1000_ran     212µs ± 1%      12µs ± 1%  -94.53%  (p=0.000 n=9+8)

name                   old alloc/op   new alloc/op   delta
EncodeFloats/10_seq       0.00B          0.00B          ~     (all equal)
EncodeFloats/10_ran       0.00B          0.00B          ~     (all equal)
EncodeFloats/100_seq      0.00B          0.00B          ~     (all equal)
EncodeFloats/100_ran      0.00B          0.00B          ~     (all equal)
EncodeFloats/1000_seq     0.00B          0.00B          ~     (all equal)
EncodeFloats/1000_ran     0.00B          0.00B          ~     (all equal)

name                   old allocs/op  new allocs/op  delta
EncodeFloats/10_seq        0.00           0.00          ~     (all equal)
EncodeFloats/10_ran        0.00           0.00          ~     (all equal)
EncodeFloats/100_seq       0.00           0.00          ~     (all equal)
EncodeFloats/100_ran       0.00           0.00          ~     (all equal)
EncodeFloats/1000_seq      0.00           0.00          ~     (all equal)
EncodeFloats/1000_ran      0.00           0.00          ~     (all equal)

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