eal: rename IPC sync request to pending request

eal: rename IPC sync request to pending request

Originally, there was only one type of request which was used
for multiprocess synchronization (hence the name - sync request).

However, now that we are going to have two types of requests,
synchronous and asynchronous, having it named "sync request" is
very confusing, so we will rename it to "pending request". This
is internal-only, so no externally visible API changes.

Suggested-by: Jianfeng Tan <jianfeng.tan@intel.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Jianfeng Tan <jianfeng.tan@intel.com>