Refactor: attrd: separate election-related code into own source file

Improves ability to follow code

Before:

116 pacemaker-attrd.h
399 pacemaker-attrd.c

1328 attrd_commands.c

After:

121 attrd_elections.c
126 pacemaker-attrd.h
396 pacemaker-attrd.c

1271 attrd_commands.c