Don't Set a Default Value on PostgreSQL SERIAL Fields

Fixes: https://github.com/doctrine/dbal/issues/2906

Serial fields already have a default value. By opting out of `notnull`
serial fields will just set the next value in the sequence.

Postgres has a `SERIAL` data type to allow shorthand for
`nextval('sequence_name')`, but DDL like this:

    CREATE TABLE example (id SERIAL)

Cannot be generated in the DBAL schema APIs.

Default value declaration for immutable types

Fixes: https://github.com/doctrine/dbal/issues/2859

Fix BC-break regarding JsonArrayType

Fixes: https://github.com/doctrine/DoctrineBundle/issues/696
Fixes: https://github.com/doctrine/dbal/issues/2880

I know, this is REALLY nasty but that was the only way I've found to
make it work properly.

And then we don't need to wait till v3 to not require comments on the
JsonType.

On 31d01283 we made JsonType the default
one, which caused some unexpected results with the comparator.

This tests validates that if users have a JSON column but are using the
JsonArrayType we'll just add a comment to the column.

Reference: https://github.com/doctrine/dbal/pull/2782

Improve build configuration

Also disabling some build failing conditions that were causing confusing
instead of helping.

Proofread types doc

It seems to be the case in the rest of this document.

Tenant documentation typos

Doc typos and grammar

fixed typo into security documentation

Add iterators for non pdo drivers.

statements in a rewindable iterator. Skipped the test for iterating
multiple times with a message regarding that iterators are non
rewindable.

Only added to mysqli for now, can add to other drivers (assuming they
support reseting the pointer in some way) after feedback.

Test fails because it's not possible to reuse a generator in this way.

I don't really understand the mysqli driver too much and the changes
break the tests so undo using an iterator for this particular driver.

Allows result sets to be iterated over without running out of memory on
large data sets.

Style improvements