INT-2562: JPA: Add 'flush' Operation

JIRA: https://jira.springsource.org/browse/INT-2562

INT-2562: Add 'flush-size' & 'clear-on-flush' attrs

INT-2562: Documentation

INT-2562: Polishing

INT-2562: Addressing PR's comments

INT-2562: Fix typos and JavaDocs
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