These new functional indexers provide a few advantages:
1. enterprise differences can be isolated to a single function (the
indexer function), making code easier to change
2. as a consequence of (1) we no longer need to wrap all the calls to
Txn operations, making code easier to read.
3. by removing reflection we should increase the performance of all
operations.
One important change is in making all the function signatures the same.
https://blog.golang.org/errors-are-values
An extra boolean return value for SingleIndexer.FromObject is superfluous.
The error value can indicate when the index value could not be created.
By removing this extra return value we can use the same signature for both
indexer functions.
This has the nice properly of a function being usable for both indexing operations.
By using a new pattern for more specific indexes. This allows us to use
the same index for both service checks and node checks. It removes the
abstraction around memdb.Txn operations, and isolates all of the
enterprise differences in a single place (the indexer).
registerSchema creates some indirection which is not necessary in this
case. newDBSchema can call each of the tables.
Enterprise tables can be added from the existing withEnterpriseSchema
shim.
Using withEnterpriseSchema() we can apply any enterprise schema changes
with a single shim, removing the need to duplicate all of the table
definitions.
Also move all the catalog schemas to a new file to shrink catalog.go a bit.
