Support Table mixin columns

This is essentially the same as #1833, #1835, #1839 but will be done in the framework of #2790 as a follow-on once that is merged.

The starting point here will be fed00dc5f6d6d9529ed8f073c95346eb176fdfa6, which *removed* the proof-of-concept changes showing that mixin columns would be straightforward in the new column-based Table structure.   In this issue (and eventual PR) we can define the right way to support mixin columns like Quantity, Time, and SkyCoord.  The original proof-of-concept used a some simple class / instance variables, but @mhvk has proposed using an abstract base class.

Closes #1833
Closes #1835
Closes #1839
In the current implementation it isn't (or at least should not be) possible to merge mixin columns.  Merging happens for join key columns (which cannot be mixins) and for vstack (which is currently not implemented for mixins).  In any case it seems that an exception will occur at the point of trying to make the new merged Quantity with inputs that have inconsistent units.  For instance `insert` fails appropriately, as well indexed setting:
```
In [10]: print(t1['q'])
[ 1.  2.] m

In [11]: t1['q'].insert(1, 3*u.m)
Out[11]: <Quantity [ 1., 3., 2.] m>

In [12]: t1['q'].insert(1, 3*u.s)
---------------------------------------------------------------------------
UnitsError                                Traceback (most recent call last)
```