transactional schema middleware

[bamarco]

No description provided.

[bamarco]

The main purpose is to be able to use a transaction like

[{:db/ident :my.attr 
  :db/valueType :db.type/ref}] 

and have it recognized in datahike as part of the schema (which stores schema as a plain map).

It also enables all sorts of middleware for transactions which can allow for customization based on different performance needs.

I've decided against using transducers to replace db/transact-tx-data because db/explode needs recursion anyways and to use transducers I needed to create a bunch of Tempid objects which I'm assuming would hurt performance more than would be gained by using transducers. There may be asynchronous contexts where this will be desirable however. A halfway implementation can be found here: https://github.com/bamarco/datascript/tree/modular

This patch is a little more fully formed than tonsky/datascript#234 with some tests.

The efficiency could be sped up if schema changes did not require re-creating the indexes. The next step there would be completing the schema validation function.