Created 2020-08-28T110259.rxp on branch 8.0.

subject: Pike 8.0: Concurrent.Promise
from: 6a692a5c
to: 390d6181
originator: Henrik Grubbström <grubba@roxen.com>
depends: 2020-08-21T103318
restart: true

Multiple changes:

• Concurrent.AggregateState: Added CAVEAT LECTOR.

• Concurrent.Promise: Improved behavior for CONCURRENT_DEBUG.

Don't store raw backtraces, they may contain references to stuff
that the user doesn't expect to get extra references.

• Concurrent.Future: Added try_get().

• Concurrent.Future: Added query_{success,failure}_callbacks().

• Concurrent.AggregateState: Protect create().