Make sl_event_lock locking work with try blocks.

The LOCK sl_event_lock must be before any queries/SQL in the
transaction.  In a try lock where multiple slonik commands
are done as part of the same transaction this causes issues.
The solution to this is that if in a try block then obtain
sl_event_lock when a transaction on the dbconn is opened

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=92f7deec97fd86d44f079bf6efea98443237ce54

Modified Files
--------------
src/slonik/dbutil.c |   47 ++++++++++++++++++++++++++++------
src/slonik/slonik.c |   68 ++++++++++++++++++++++++++++++---------------------
src/slonik/slonik.h |    3 +-
3 files changed, 80 insertions(+), 38 deletions(-)