dedupe x-axis values more carefully

[spalger]

Fixes #4670

In #4613 the range buckets are no longer represented by a string, but by an object. This enhances support for such keys by:

• Always returning the same/=== key for buckets which are a part of the same range
• Supporting string serialization (for d3 domain support)
• Using a map, rather than an object, for deduping values in vislib/components/zero_injection/uniq_key.js

[w33ble]

Fix works, LGTM.

[lukasolson]

1. Create a bar chart
2. Create a range x-axis (or split bars) aggregation on a scripted field
3. Click on one of the bars to filter

I get the following error:

Error: Request to Elasticsearch failed: {"error":{"root_cause":[{"type":"expression_script_compilation_exception","reason":"Failed to parse expression: (abs(doc['amount'].value))>=gte && (abs(doc['amount'].value))<lt && (abs(doc['amount'].value))function toString() { [native code] }toString && (abs(doc['amount'].value))undefinedvalue","stack_trace":"ExpressionScriptCompilationException[Failed to parse expression: (abs(doc['amount'].value))>=gte && (abs(doc['amount'].value))<lt && (abs(doc['amount'].value))function toString() { [native code] }toString && (abs(doc['amount'].value))undefinedvalue]; nested: ParseException[ missing 'function' at position (94).]; nested: MissingTokenException;\n\tat org.elasticsearch.script.expression.ExpressionScriptEngineService.compile(ExpressionScriptEngineService.java:98)\n\tat

I tried this on master (with split bars, cuz obviously x-axis isn't working with range) and didn't get the same error.

[spalger]

@lukasolson at some point the bucket key is passed as params and itterated as though every key is a parameter name. This seems fair to me, so I made #toString() non-enumerable by creating a little RangeKey class. This also presented an issue with the way that scripted fields are formatted in the filter bar, which I took care of really quick.

[lukasolson]

LGTM.