Attach keys to the strategy actions

[kabasset]

In order to allow removing or copying actions, they should be attached a key (e.g. an encapsulated std::list::iterator, which gets not invalidated by adding or removing actions). A special value could be assigned to compression.

[kabasset]

Example:

Strategy s;
auto id = s.append(ValidateChecksums());
s.append(Compress<HCompress>());
auto action = s[id];
s.erase(id);

Should it even be a container, i.e. with an actual iterator?