SL-14399: Ditch overflow queue LLViewerAssetStorage::mCoroWaitList.

mCoroWaitList was introduced to prevent an assertion failure crash:
LLCoprocedureManager never expects to fill LLCoprocedurePool::mPendingCoprocs
queue. The queue limit was arbitrarily set to 4096 some years ago, but in
practice LLViewerAssetStorage can post way more requests than that.

LLViewerAssetStorage checked whether the target LLCoprocedureManager pool's
queue looked close to full, and if so posted the pending request to its
mCoroWaitList instead. But then it had to override the base LLAssetStorage
method checkForTimeouts() to continually check whether pending tasks could be
moved from mCoroWaitList to LLCoprocedureManager.

A simpler solution is to enlarge LLCorpocedureManager::DEFAULT_QUEUE_SIZE, the
upper limit on mPendingCoprocs. Since mCoroWaitList was an unlimited queue,
making DEFAULT_QUEUE_SIZE "very large" does not increase the risk of runaway
memory consumption.