Fix copylock when use sync.Pool in clickhouse.Pool

[fengberlin]

In the conn_http.go, line 48, the clickhouse.Pool contains a sync.Pool, but according to sync.Pool doc:

A Pool must not be copied after first use

That is because sync.Pool contains noCopy. So to avoid copied after use, we should wrap a *sync.Pool in clickhouse.Pool:

type Pool[T any] struct {
    pool *sync.Pool
}

[CLAassistant]

All committers have signed the CLA.