limit the max size of HttpClient waiter queue and the max outstanding requests of each connection to avoid OOM under high load.
check the outstanding request size while return connection to pool
move connection max outstanding requests property from pool to connection itself
fix syntax error
add more comments
introduce type ConnectionPoolTooBusyException to indicate that the connection pool is too busy to handle more requests
skip returning a fully occupied connection to connection pool in DefaultHttpClient. It will prevent a fully occupied conn from assigning more requests or returning to available queue.
Merge pull request #5 from wangjunbo924/master

able to limit the size of requests in waiter queue and pipeline queue