Configurable Transaction Isolation

[lvca]

I double-checked the code and the docs about the transaction batch are not correct. That piece was coming from OrientDB which allowed some control over isolation. With ArcadeDB the level is always Read committed, which means dirty reads are not possible, but you can get Nonrepeatable Read and Phantom Read. It would be pretty easy to support both REPEATABLE_READ and SERIALIZABLE, I'm opening an issue for that.

Originally posted by @lvca in #994 (comment)

[lvca]

Working more deeply on this issue, we can't provide Serializable without modifying a large part of the code. The main reasons are:

• with serializable, when the transaction starts can't see any new changes committed after the transactions started. This means the database should keep track of the running transactions and therefore old copies of pages until the transactions end.
• serializable is very rarely used in production, even with RDBMSs, because the overhead is so big that is simply not worth it for most of the use cases.