Refactor API

[kelindar]

This PR introduces major breaking change in the API by removing the existing cursor and changing the way how the columns are accessed during transactions. Since columnar storage is intended to work on ranges of values, this speeds up range queries by around ~20-40%.

In order to access the results of the iteration, prior to calling Range() method, we need to first load column reader(s) we are going to need, using methods such as txn.String(), txn.Float64(), etc. These prepare read/write buffers necessary to perform efficient lookups while iterating.

In the example below we select all of the rogues from our collection and print out their name by using the Range() method and accessing the "name" column using a column reader which is created by calling txn.String("name") method.

players.Query(func(txn *Txn) error {
	names := txn.String("name") // Create a column reader

	return txn.With("rogue").Range(func(i uint32) {
		name, _ := names.Get()
		println("rogue name", name)
	})
})

Similarly, if you need to access more columns, you can simply create the appropriate column reader(s) and use them as shown in the example before.

players.Query(func(txn *Txn) error {
	names := txn.String("name")
	ages  := txn.Int64("age")

	return txn.With("rogue").Range(func(i uint32) {
		name, _ := names.Get()
		age,  _ := ages.Get()

		println("rogue name", name)
		println("rogue age", age)
	})
})