Refactor failover().

Refactor failover().

failover() was too large and hard to maintain. By refactoring it, the
size is reduced from 798 lines to 215 lines.  It is now splitted into
following subroutines. failover() just calls them.

static int handle_failback_request(FAILOVER_CONTEXT *failover_context, int node_id);
static int handle_failover_request(FAILOVER_CONTEXT *failover_context, int node_id);
static void kill_failover_children(FAILOVER_CONTEXT *failover_context, int node_id);
static void exec_failover_command(FAILOVER_CONTEXT *failover_context, int new_main_node_id, int promote_node_id);
static int determine_new_primary_node(FAILOVER_CONTEXT *failover_context, int node_id);
static int exec_follow_primary_command(FAILOVER_CONTEXT *failover_context, int node_id, int new_primary_node_id);
static void save_node_info(FAILOVER_CONTEXT *failover_context, int new_primary_node_id, int new_main_node_id);
static void exec_child_restart(FAILOVER_CONTEXT *failover_context, int node_id);
static void exec_notice_pcp_child(FAILOVER_CONTEXT *failover_context);