Rename `shard` to `pool`

The server's shard feature has been renamed to pool. This patch reflects
that change in the client library. The patch keeps old `shard*` methods
and functions for backwards compatibility and it also raises a
deprecation warning whenever they are used. These functions will be
removed right after k-1

Change-Id: I842bfbcf075845750bc4fff345086636cc775371
Closes-bug: #1348196