Add scaled_float.

[jpountz]

This is a tentative to revive #15939 motivated by elastic/beats#1941.
Half-floats are a pretty bad option for storing percentages. They would likely
require 2 bytes all the time while percentages really don't need more than one
byte.

So this PR exposes a new scaled_float type that requires a scaling_factor
and internally indexes value*scaling_factor in a long field. Compared to the
original PR it exposes a lower-level API so that the trade-offs are clearer and
avoids any reference to fixed precision that might imply that this type is more
accurate (actually it is less accurate).

In addition to being more space-efficient for some use-cases that beats is
interested in, this is also faster that half_float unless we can improve the
efficiency of decoding half-float bits (which is currently done using software)
or until Java gets first-class support for half-floats.

[jpountz]

To give more detailed information about why half floats are not enough, here is a table that gives disk usage for storing 10M random floats between 0 and 1 depending on the mapping:

Mapping	Points disk usage (kB)	Doc values disk usage (kB)	Total
float	49728	34180	83908
half float	26560	19532	46092
scaled float (factor=4000)	25744	14652	40396
scaled float (factor=100)	13044	9768	22812

I chose 4000 and 100 as scaling factors because 4000 means 0.025% accuracy, which is better than what a half float can do for this particular use case (floats between 0 and 1) yet requires less disk, and 100 because I suspect it would be enough for many metrics like cpu utilization with its 1% accuracy.

Of course this is not a good benchmark since this is fake data, but given how points and doc values work this simulates the worst case and real data could expect even better disk utilization.

[martijnvg]

Just a question: would it be possible to extend from LongFieldMapper? Would be nice to have some code reuse.

[jpountz]

I thought about it when working on this PR but in the end it made things more complicated since this mapper partially needs to behave as a long field and as a double field.

[martijnvg]

Cool, I can see how this can complicate things, was just hoping that this code reuse would be a low hanging fruit.

[jpountz]

Updated numbers with https://issues.apache.org/jira/browse/LUCENE-7371:

Mapping	Points disk usage (kB)	Doc values disk usage (kB)	Total
float	40312	34180	74492
half float	23092	19532	42624
scaled float (factor=4000)	22792	14652	37444
scaled float (factor=100)	12984	9768	22752