Feat: Remove byte slice in ColumnAlloc

[j2gg0s]

If we don't reuse ColumnaAlloc's byte slice, we can remove it and simple logic.

Benchmark with current version

➜  pool git:(v10) ✗ go test ./... -bench=BenchmarkColumnAlloc
Running Suite: pool
===================
Random Seed: 1604545377
Will run 23 of 23 specs

•••••••••••••••••••••••
Ran 23 of 23 Specs in 0.419 seconds
SUCCESS! -- 23 Passed | 0 Failed | 0 Pending | 0 Skipped
goos: darwin
goarch: amd64
pkg: github.com/go-pg/pg/v10/internal/pool
BenchmarkColumnAlloc-8           5052452               234 ns/op
PASS
ok      github.com/go-pg/pg/v10/internal/pool   2.151s

Benchmark with new version

➜  pool git:(feat-simple-column-alloc) go test ./... -bench=BenchmarkColumnAlloc
Running Suite: pool
===================
Random Seed: 1604545352
Will run 23 of 23 specs

•••••••••••••••••••••••
Ran 23 of 23 Specs in 0.379 seconds
SUCCESS! -- 23 Passed | 0 Failed | 0 Pending | 0 Skipped
goos: darwin
goarch: amd64
pkg: github.com/go-pg/pg/v10/internal/pool
BenchmarkColumnAlloc-8           4844086               237 ns/op
PASS
ok      github.com/go-pg/pg/v10/internal/pool   2.135s

[vmihailenco]

Thanks