[Cbc] Cbc segfault after many hours of CPU time

[John Forrest]

Status report on segfault.

Unable to reproduce with debug version.

Have reproduced segfaults three times with optimized version.  It is 
always to do with multi-threading.  Once I could see something I didn't 
like so have modified that.  However the other two times it was not 
obvious.  Looking at registers and disassembled code I could see 
segfault but going from a few instructions back it should have worked.  
So classic overwriting due to threads.  But I can't see what is wrong 
with locking/unlocking threads to stop overwriting. Will continue 
looking slowly.

However if you use -thread 104 instead of 4 that switches on 
deterministic parallel.  This is not quite as efficient (it would be a 
lot better if the effort per node was better determined e.g. as in 
Cplex's"ticks") but does not have same problem.  That has been running a 
long time without a problem (>63 million nodes).

Using my (not in svn yet) trunk and throwing every cut at problem I can 
prove solution of  499243.8 is optimal after 23 million nodes.

However looking at problem I tried another trivial experimental feature 
on problem.  There may be bugs, but I don't think so.  That took 15,368 
nodes and 54 seconds.

John Forrest