Separate Block into AbstractBlock, Block and SystemBlock

Separate Block into AbstractBlock, Block and SystemBlock

This commit splits the existing Block class into AbstractBlock, Block
and SystemBlock.

Before this patch, the Block class represents several types of
blocks, which can be separated into blocks stored in the database,
and temporary blocks created by the system. These are now
represented by Block and SystemBlock, which inherit from
AbstractBlock.

This lays the foundations for:
* enforcing block parameters from multiple blocks that apply to a
user/IP address
* improvements to the Block API, including the addition of services

Breaking changes: functions expecting a Block object should still
expect a Block object if it came from the database, but other
functions may now need to expect an AbstractBlock or SystemBlock
object. (Note that an alternative naming scheme, in which the
abstract class is called Block and the subclasses are DatabaseBlock
and SystemBlock, avoids this breakage. However, it introduces more
breakages to calls to static Block methods and new Block
instantiations.)

Changes to tests: system blocks don't set the $blockCreateAccount or
$mExipry block properties, so remove/change any tests that assume
they do.

Bug: T222737
Change-Id: I83bceb5e5049e254c90ace060f8f8fad44696c67