Inconsistent synchronization around odbc sync handling

There is a narrow race condition when an odbc message is followed by sync - the isExecuting check is supposed to prevent the sync until the execution is complete, but is not synchronized.

The result can be an exception in the pg client:

java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(ArrayList.java:653)
at java.util.ArrayList.get(ArrayList.java:429)
at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1825)
at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:255)

As the pendingExecuteQueue will have been cleared by the early response to sync.