Adjust debug log for caches being used

I always found out the debug message listing the cache being in use to
be slightly confusing:

    MemcachedPeclBagOStuff[main] MemcachedPeclBagOStuff[message]
    MemcachedPhpBagOStuff[parser]

Swap the class name and identifier to output:

    main: MemcachedPeclBagOStuff, message: MemcachedPeclBagOStuff,
    parser: MemcachedPhpBagOStuff

Change-Id: If069dec70eb31ed2dafe0e88ec4d4653005ff7e3