[Orc][RPC] Lock the pending results data structure when installing new result

handlers, make abandonPendingResults public API.

This should make installing asynchronous result handlers thread safe.

The abandonPendingResults method is made public so that clients can disconnect
from a remote even if they have asynchronous handlers awaing results from that
remote. The asynchronous handlers will all receive "abandoned result" errors as
their argument.

llvm-svn: 291399