Add support for customising ThreadPool queue size

[lloydmeta]

Closes #175

• Add the model field + db persistence field w/ tests
• Add the field to transformation from the JSON model (update tests)
• Add the field to the ThreadPool edit form
• Add support for reading the edit and create POST request

[mauhiz]

:+1: (I know you are going to need 2 of these)

[lloydmeta]

@mauhiz cheers man XD (now go back to your forking of core apache projects!)

[xevix]

LGTM, merging. :+1: . There, that's two, just for @mauhiz .