INT-3006: Resolve 'maxSubscribers' from INT Props

JIRA: https://jira.springsource.org/browse/INT-3006

* Add integration properties `channels.maxUnicastSubscribers` and `channels.maxBroadcastSubscribers`
with default value to `Integer.MAX_VALUE`
* Add SpEL value resolution for those default values from parsers
* Deprecate similar properties from `ChannelInitializer`

INT-3006 `maxSubscribers` from channels' onInit()

* Move default init for `maxSubscribers` to the channels' `onInit()`
from `integrationProperties`, not from parsers
* extract `default` `integrationProperties` from static block in the `IntegrationProperties`
and use them as `default` on `IntegrationContextUtils#getIntegrationProperties`
* Add bean definition expression building to get the default values for `integration properties`
on bean building phase.

INT-3006: Add `taskScheduler.poolSize` property

INT-3006: Polishing

spring-integration-core/src/test/resources/META-INF/spring.integration.properties

@@ -1,2 +1,5 @@
 #channels.autoCreate=false
+#channels.maxUnicastSubscribers=1
+#channels.maxBroadcastSubscribers=1
 messagingTemplate.lateReply.logging.level=error
+taskScheduler.poolSize=20