Collection support in pillowfight

It would be very useful to enhance pillowfight with collection awareness to assist testing and benchmarking of the server.

Note cbworkloadgen supports collections, at a minimum we should copy what cbworkloadgen does.

cbworkloadgen was changed to accept two new CLI arguments.

1. -s
2. -c

The -s option allows the client to specify a custom separating character which will be used in forming keys <prefix><separator><random>.

The -c option allows the client to specify collection names, these really just describe the prefix. When the -c option is specified cbworkloadgen adds "collections" to its HELLO message. Note that cbworkloadgen allows many -c inputs, each one adding another prefix to the list allowed.

Examples:

> cbworkloadgen <usual options> -s : -c beer

Will generate only beer:<random> keys

> cbworkloadgen <usual options> -s : -c beer -c brewery

Using many -c will alternate in generating beer:<random>, brewery:<random> keys

Basically the key generation will round-robin through the list of collections so we evenly write/read to the list specified.