HttpServerExchange accumulation with async requests on EAP 7

Issue

• We see a growing number of io.undertow.server.HttpServerExchanges retained like so with async requests leading to an OutOfMemoryError:

Class Name                                                                                                      | Ref. Objects | Shallow Heap | Ref. Shallow Heap | Retained Heap
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
org.xnio.nio.WorkerThread @ 0x5000732b8  default I/O-11 Thread                                                  |          269 |          152 |            47,344 |         8,464
|- <Java Local>, selector sun.nio.ch.EPollSelectorImpl @ 0x5022a3208                                            |          262 |           72 |            46,112 |        19,464
|  '- fdToKey java.util.HashMap @ 0x5022eb8c0                                                                   |          262 |           48 |            46,112 |        19,200
|     '- table java.util.HashMap$Node[512] @ 0x6359c80f0                                                        |          262 |        2,064 |            46,112 |        19,152
|        |- [30] java.util.HashMap$Node @ 0x7744cb260                                                           |            2 |           32 |               352 |            96
|        |  |- next java.util.HashMap$Node @ 0x265839df8                                                        |            1 |           32 |               176 |            48
|        |  |- value sun.nio.ch.SelectionKeyImpl @ 0x6e560f040                                                  |            1 |           40 |               176 |            40
|        |  |  '- attachment org.xnio.nio.NioSocketConduit @ 0x7139e2638                                        |            1 |           64 |               176 |            64
|        |  |     '- writeReadyHandler io.undertow.protocols.ssl.SslConduit$SslWriteReadyHandler @ 0x584ed0a20  |            1 |           24 |               176 |            40
|        |  |        '- delegateHandler org.xnio.conduits.WriteReadyHandler$ChannelListenerHandler @ 0x572e48ec8|            1 |           16 |               176 |            16
|        |  |           '- channel org.xnio.conduits.ConduitStreamSinkChannel @ 0x771e5c8b0                     |            1 |           32 |               176 |           256
|        |  |              '- conduit io.undertow.conduits.ChunkedStreamSinkConduit @ 0x761d02ed8               |            1 |           56 |               176 |           224
|        |  |                 '- attachable io.undertow.server.HttpServerExchange @ 0x6abeb1408                 |            1 |          176 |               176 |     3,091,280