Online DDL - stage 1

[losfair]

Related: #70

This PR implements a restricted form of online DDL. It permits concurrent read-only DML when a schema migration task is in progress.

Currently you can already do non-blocking DDL with regular transactions, but that has a few limitations:

• Schema migrations in a large database usually take a long time, and are likely to be indefinitely starved by concurrent small transactions in a busy system.
• A transaction can write at most 50,000 pages of data. This forces the application developer to split a schema migration into multiple smaller transactions, and the application may see inconsistent data in between.

This PR introduces single-writer lock, which when enabled permits only a single writer, the schema migration task, to write to the database. All other tasks see the latest consistent snapshot before the lock.

[losfair]

It works, but still needs a careful review for correctness.

So much concurrency here.