TypeDB 3.0: remove sessions in favor of 3 transaction types

[flyingsilverfin]

Problem to Solve

TypeDB's connection model introduces needless complexity, as well as some states of questionable use (such as a schema session + read transaction).

Proposed Solution

We will simplify to use 3 transaction types directly:

• schema - which allows schema and data manipulation, to be used to defining schema and schema+data migrations.
• read - allow reading the data + schema in a snapshot
• write - allows writing data and reading data + schema in a snapshot.

While a schema transaction is open, data write transactions are blocked.