Faster path conn pool with available conns

This dropped us from about 824 ns/op with 3 allocations of about 400
bytes to 180 ns/op and no allocations.

Before:
BenchmarkBestCaseCPGet   2000000  848 ns/op   369 B/op   3 allocs/op

After:
BenchmarkBestCaseCPGet  10000000  180 ns/op     0 B/op   0 allocs/op