sql: use a string builder in decodeCopy

[maddyblue]

For returning strings, a string builder is better than a bytes buffer.

name                  old time/op    new time/op    delta
DecodeCopySimple-12     10.8ns ± 1%    10.6ns ± 1%   -2.03%  (p=0.024 n=5+5)
DecodeCopyEscaped-12     249ns ± 1%     178ns ± 2%  -28.71%  (p=0.008 n=5+5)

name                  old alloc/op   new alloc/op   delta
DecodeCopySimple-12      0.00B          0.00B          ~     (all equal)
DecodeCopyEscaped-12     96.0B ± 0%     40.0B ± 0%  -58.33%  (p=0.008 n=5+5)

name                  old allocs/op  new allocs/op  delta
DecodeCopySimple-12       0.00           0.00          ~     (all equal)
DecodeCopyEscaped-12      2.00 ± 0%      2.00 ± 0%     ~     (all equal)

Release note: None

[cockroach-teamcity]

This change is 

[maddyblue]

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)