Add recursive idb support

[jingjingwang]

This PR adds support for a new syntax: a do-until-convergence loop containing a list of special MyriaL statements. The loop terminates when there will be no update to any relation. Examples are those newly added optimizer tests.

[jingjingwang]

I expect myself adding some more checks to limit what can be put inside of the do-until-convergence loop, but would like to get the main changes being reviewed first.

For 059673c, I'm not sure if it's the best design. I would like to add a MIN aggregate that takes a list of column indices to compare two tuples in that order, however the current MIN is a binary expression. I added a new aggregate called MULTIMIN instead with less support (e.g. fakedb) than MIN, but maybe extending MIN to an nary expression is another option? Would like to hear your thought.

[coveralls]

Coverage decreased (-0.2%) to 91.822% when pulling 22f8ec9bec9b4b0f4c8d9d15588eb14a97d7626b on add_recursive_idb_support into 55653947635ebeeed48c975f6a2c341a97b8f116 on master.

[coveralls]

Coverage decreased (-0.06%) to 91.898% when pulling d86a4b0482a8e505ea5764e21e73b70abd9f966e on add_recursive_idb_support into 6d75be5d6b3d5bc0a490428bb2938c67039b7330 on master.

[jingjingwang]

I've addressed most comments except for a few pending discussions:

• The Zeroary MyriaConsumer for breaking infinite optimization
• MIN vs. MULTIMIN and should they use named attribute (and what if we don't know the name)
• Join pull order: global or within do-until-convergence only, where to specify

We can discuss them in person if necessary.

[coveralls]

Coverage decreased (-0.06%) to 91.898% when pulling da94a7f717c219aa087ba47d96fe4bd8061f299c on add_recursive_idb_support into 6d75be5d6b3d5bc0a490428bb2938c67039b7330 on master.

[coveralls]

Coverage decreased (-0.1%) to 91.839% when pulling 81e9e1ed13a47f4218c4bb8008acc9e697d9902d on add_recursive_idb_support into 6d75be5d6b3d5bc0a490428bb2938c67039b7330 on master.

[coveralls]

Coverage decreased (-0.05%) to 91.912% when pulling 718157d9cc4d256d3145d33298ea0d324d28e0dc on add_recursive_idb_support into 6d75be5d6b3d5bc0a490428bb2938c67039b7330 on master.

[coveralls]

Coverage decreased (-0.05%) to 91.917% when pulling cca8d107132f4df189a3ac5f9c28c0a0367a4d74 on add_recursive_idb_support into 6d75be5d6b3d5bc0a490428bb2938c67039b7330 on master.

[coveralls]

Coverage decreased (-0.05%) to 91.917% when pulling 7f0716211eb421c2c2772d88258b40bf9cab35ab on add_recursive_idb_support into 6d75be5d6b3d5bc0a490428bb2938c67039b7330 on master.

[jingjingwang]

Goes with https://github.com/uwescience/myria/pull/877.