Dima/abstract execution base

[dorezyuk]

Hi guys,

The testing goes on. In this pr its about the abstract_execution_base.

MInor adjustments

• pass and get strings by const ref instread of by value

Major Fixes

• we never joined threads when destructing the class.
• we never checked if the thread_ actually was stopped when starting it.
• I get a segfault if the instance would be destructed right after calling start(). The exact reason for this is rather complex (== not sure if I understand it fully myself) but converting the pure virtual methods to just virtual methods solves it.
• we never reset cancel_ and other execution related data. As far as i can see this would render the whole class unusable if the user would cancel an action once.

Best, Dima

[corot]

@dorezyuk, can u rebase with latest master?

[dorezyuk]

rebased