[Lens] Build index pattern field map for O(1) access

[flash1293]

Currently the Lens-specific IndexPattern type only contains an array of field which is used in a lot of places to find a field by name.

This is problematic if the index pattern has a lot of fields.

To improve performance, the loader should expose the data index pattern object already providing this functionality by getFieldByName.

By completely removing the local index pattern type we can also remove redundancy and a bit of the complexity of Lens.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

I agree this would be nice, but also want to point out that this is another place where we're duplicating effort in Lens + the Index Pattern service. Would be nice if we could start removing some of the Lens-specific handling, and it might even improve performance if we did. As a reminder this is tech debt because Lens was started before the index pattern service was easily shareable.

[flash1293]

Good point @wylieconlon - I agree it's probably a better approach to move away from the Lens-specific index pattern type and just use data index pattern objects directly. I will change the description to reflect this.