Optimize keys collection Transient for faster GET's.

The Transient pointer is now to an item, so no longer need to parse
values and also making less garbage.

On my macbook pro, GET-only memcachetest seems to have no more GC
pauses and GOGCTRACE stats look more stable...

    /memcachetest -h 127.0.0.1:11210 -L 2 -l -t 8 -P 0
    Average with 8 threads
    Get operations:
         #of ops.       min       max       avg   max90th   max95th   max99th
            10000     61 us   1166 us    338 us    468 us    590 us    802 us