Adds a `$beginsWith` operator to selectors, with json and text index
support. This is a compliment / precursor to optimising `$regex`
support as proposed in #4776.

For `json` indexes, a $beginsWith operator translates into a key
range query, as is common practice for _view queries. For example,
to find all rows with a key beginning with "W", we can use a range
`start_key="W", end_key="W\ufff0"`. Given Mango uses compound keys,
this is slightly more complex in practice, but the idea is the same.
As with other range operators (`$gt`, `$gte`, etc), `$beginsWith`
can be used in combination with equality operators and result sorting
but must result in a contiguous key range. That is, a range of
`start_key=[10, "W"], end_key=[10, "W\ufff0", {}]` would be valid,
but `start_key=["W", 10], end_key=["W\ufff0", 10, {}]` would not,
because the second element of the key may result in a non-contiguous
range.

For text indexes, `$beginsWith` translates to a Lucene query on
the specified field of `W*`.

If a non-string operand is provided to `$beginsWith`, the request will
fail with a 400 / `invalid_operator` error.