ike-sa-manager: Improve scalability of half-open IKE_SA checking

ike-sa-manager: Improve scalability of half-open IKE_SA checking

This patch is based on one by Christoph Gouault.

Currently, to count the total number of half_open IKE_SAs,
get_half_open_count sums up the count of each segment in the SA hash
table (acquiring a lock for each segment).  This procedure does not scale
well when the number of segments increases, as the method is called for
each new negotiation.

Instead, lets maintain a global atomic counter.

This optimization allows the use of big values for charon.ikesa_table_size
and charon.ikesa_table_segments.