fake or mock site handler for in-memory perf tests

Unlike the sibling catalog/file site handler, this catalog/mock site
handler is in-memory only, with no file I/O, and generates documents
on the fly as needed (as opposed to pre-generating all documents
beforehand).

By default, a mock site has one pool (p0) with one bucket (b0) with
100000 items.  To use it, use "mock:" for your couchbase site path...

    ./tuqtng -couchbase mock: -pool p0

The "mock:" path can also have optional comma-separated key=val
parameters.  For example...

    ./tuqtng -couchbase mock:pools=4,buckets=3,items=10000 -pool p0

That will create pools p0, p1, p2, p3, where each pool will have
buckets b0, b1, b2, and each bucket will have items "0" to "9999".

Each item or document will look like...

    { "id": "123", "i": 123 }