Adds condition-variable-type event objects that threads can wait on and
signal to the API.  The implementation is just a wrapper around the
internal event_t interfaces.

Adds a test to the client.thread test by converting its prior hacky use of
bools and yielding to proper events.

Fixes #279

Adds condition-variable-type event objects that threads can wait on and
signal to the API.  The implementation is just a wrapper around the
internal event_t interfaces.

Adds a test to the client.thread test by converting its prior hacky use of
bools and yielding to proper events.

Fixes #279

This change improves performance and reduces memory bandwidth contention.
Access to the shared memory is handled by a ticket lock that maintains
the order of threads waiting, in an attempt to reduce skew between
thread events. A custom futex implementation had to be used because
DynamoRIO does not provide a conditional variable implementation,
and the use of the pthread library is not allowed in DR clients.

DynamoRIO#279