Rename worker threads as GIO threads.

Currently, the worker threads are named read only and read/write. This description is no longer valid with GIO since any thread can pick up task from any GIO queue.

example output from cbstats-Here ro_0 so on should be replaced with gio_0 etc.
./cbstats localhost:11210 kvstore
ro_0:backend_type: couchstore
ro_0:close: 0
ro_0:failure_get: 0
ro_0:failure_open: 1024
ro_0:numLoadedVb: 0
ro_0:open: 1024
ro_1:backend_type: couchstore
ro_1:close: 0
ro_1:failure_get: 0
ro_1:failure_open: 0
ro_1:numLoadedVb: 0