Support additional filters

[bullet-db]

Support additional filters

[akshaisarma]

Comparing to other fields requires us to address casting (see #37):

1. We should make the filter clause values, a List[String] to a List[Object] (we can support List[String] for backward compatible)
2. When we see List[String], we make it into:

{
   "value": "the string value"
}

as below.
3. The Object in the List[Object] is now a new object:

{
   "kind": "VALUE | CAST | FIELD"
   "value": "a string value as before",
   "type": "BOOLEAN, STRING, LONG, INTEGER, DOUBLE, FLOAT"
}

If the kind is:
a. VALUE or is not present, we treat it as a string as before.
b. CAST, we use type to cast the value to the requisite type
c. FIELD, we treat value as a field and extract from the object to compare it the field. If types don't match, we convert the value to the type of the other field (the LHS)

[akshaisarma]

For c, we're going to follow Java's widening conventions. If the LHS is narrower than the RHS, then the cast will fail but since it's a field also, the query can always be rewritten to swap the RHS with the LHS (making the RHS the narrower one now)

[akshaisarma]

@hbxie Let's keep this open till casting is implemented too.

[akshaisarma]

Instead of having a CAST kind, a new field called type was added instead, if when specified is used to cast