Prohibit non-IMMUTABLE functions in multi-shard SELECT queries

**Issue by [jasonmp85](https://github.com/jasonmp85)**
_Thursday Jan 22, 2015 at 23:09 GMT_
_Originally opened as https://github.com/citusdata/pg_shard/issues/60_

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Though all (non-side-effecting) functions are safe to execute in a single-shard `SELECT` query, the same is not true for multi-shard SELECTs. In particular, a function such as `random()` or `now()` would return a different value for each shard and so the query results would not represent a consistent view of the world.

This may overlap somewhat with [47](https://github.com/citusdata/pg_shard/issues/47), but that issue is more concerned with single-shard ramifications. We've noticed this bug affects multi-shard queries as well, so they'll need similar treatment.

No file or test is named. Start by locating the multi-shard SELECT validation or planning path and its handling of PostgreSQL function volatility; done means non-IMMUTABLE functions such as random() and now() are rejected for multi-shard queries, with coverage for those cases.

Written by the indexing model from the issue text.