ovsdb-idl: Plug hole in state machine.

ovsdb-idl: Plug hole in state machine.

The state machine didn't have a proper state for "not yet committed or
aborted", which meant that destroying an ovsdb_idl_txn without committing
or aborting it caused a segfault.  This fixes the problem by adding a new
state TXN_UNCOMMITTED to the state machine.

This is related to commit 79554078d "ovsdb-idl: Fix bad logic in
ovsdb_idl_txn_commit() state transitions", which fixed a related bug.

Bug #2438.