Postbacks inside postbacks

[tomasherceg]

During the execution of a postback or static command, another postback/static command can be invoked (either by a postback handler or from a JS function...).

There are several combinations that can happen (postback inside postback, postback inside a static command, and so on).

We need to further investigate it. Based on what we find, we should:

• make sure it works and is covered by tests
• disable the particular combinations if they cannot be supported

[acizmarik]

This feature seems to be working as it is already used in some projects. We should cover this behaviour with tests still.

[exyi]

Tests added in #1709. It seems to work, but note that the nested postbacks must be in a different concurrency queue (regardless of the concurrency mode).