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
INT-3217 Update Default Spring to 3.2.5

JIRA: https://jira.springsource.org/browse/INT-3217

INT-3217 Polishing

PR Comments

Also fixes some other warnings (mainly unused imports).
What's New Polishing

Typos; add a comment about HTTP paths.
INT-3218 Spring 4 Compatibility

Use `Arrays.asList()` instead of `Collections.arrayToList()` in
places where a generic type needs to be supplied by the caller.

This maintains compile-time compatibility with both Spring 3 and 4,
where Spring 4 added the generic type to the utility class.

The four occurrences where this was needed are not impacted
by the fact that `Arrays.asList()` returns an unmodifiable list.