This PR makes several fixes to computed functions and highcharts/d3fc:

• Fixes an issue where computed columns would not be displayed correctly in chart axis, both in highcharts and d3fc. This was due to computed columns no longer being present in the table schema.
• Fixes an issue where datetime computed functions calculated buckets in UTC and not local time. This caused issues with datetimes that were part of one bucket in local time, but another in UTC - i.e. 03/15/2020 23:59 EST becomes 03/16/2020 04:59 UTC.
• Distinguishes between computed_schema on the view and on the table in PerspectiveManager, as they have different signatures. A future improvement would be renaming Table.computed_schema to something like Table.get_computed_schema, as the table method receives a set of computed columns and calculates their output types without actually creating the columns themselves.
• Cleans up the internals of PerspectiveManager and returns an error message if table.delete() is called on a remote table. Previously, table.delete() would fail due to code flow reasons, which prevented the method from being called but is opaque. This provides a transparent error message and explanation.
• Fixes the last by index aggregate to use psp_okey instead of psp_pkey in the aggspec, as psp_pkey does not exist in some of the intermediate data structures used during a live update. Additionally, rename AGGTYPE_LAST to AGGTYPE_LAST_BY_INDEX, as the last aggregate uses AGGTYPE_LAST_VALUE.