pgsql: Add support for blocked commands in isolationtester

Add support for blocked commands in isolationtester

This enables us to test that blocking commands (such as foreign keys
checks that conflict with some other lock) act as intended.  The set of
tests that this adds is pretty minimal, but can easily be extended by
adding new specs.

The intention is that this will serve as a basis for ensuring that
further tweaks of locking implementation preserve (or improve) existing
behavior.

Author: Noah Misch

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/846af54dd5a77dc02feeb5e34283608012cfb217

Modified Files
--------------
src/test/isolation/README                     |   13 ++
src/test/isolation/expected/fk-contention.out |   17 ++
src/test/isolation/expected/fk-deadlock.out   |   67 ++++++++
src/test/isolation/expected/fk-deadlock2.out  |  107 ++++++++++++
src/test/isolation/isolation_schedule         |    3 +
src/test/isolation/isolationtester.c          |  215 +++++++++++++++++++++----
src/test/isolation/specs/fk-contention.spec   |   19 +++
src/test/isolation/specs/fk-deadlock.spec     |   54 ++++++
src/test/isolation/specs/fk-deadlock2.spec    |   59 +++++++
9 files changed, 524 insertions(+), 30 deletions(-)