Use AI_CA_LEADER_SHARES_KEEP macro name

This used to be called AI_CA_PASSIVE_LEADER_SHARES_KEEP, renamed in BfW
1.11.2.  The old macro name still works, but is deprecated.
Use leader_shares_keep for aspect key name

This used to be passive_called leader_shares_keep, renamed in BfW
1.11.2.  The old macro name still works, but is deprecated.
Use ai_default_rca to identify RCA AI

This used to be called testing_ai_default, renamed in 1.11.2.
Add  BfW 1.11.2 requirement message to campaign description

The campaign won't even start with earlier versions, so there's no need
to put it into the opening story text.
Add function ai_helper.print_ts()

Same as print(), but prints arguments preceded by a time stamp in
seconds.
Add function ai_helper.print_ts_delta()

Same as ai_helper.print_ts(), but also prints time elapsed since the
time passed in the first argument.
battle_calcs.get_attack_combos_subset(): use wesnoth.get_time_stamp() instead of os.clock()

The former appears to be about 10 times faster than the latter.  It
also has an additional significant digit, although that doesn't matter
for this application.