make completion managers responsible for their own caches

make completion managers responsible for their own caches
- move cache out of CompletionDomainHandler into CompletionManager
- make CompletionManager longer lived
- move context and source fields out of DartCompletionManager and into superclass
- encapsulate information related to a specific request in new CompletionRequest rather than in the manager
- remove request specific information from CompletionManager creation
- use same ContextManager instance for consecutive requests on the same context/source pair

BUG=
R=scheglov@google.com

Committed: https://code.google.com/p/dart/source/detail?r=42157

[danrubel, 2014-12-06 19:32:38]

danrubel@google.com changed reviewers:
+ scheglov@google.com

[scheglov, 2014-12-07 02:12:31]

LGTM

[danrubel, 2014-12-07 19:32:07]

Committed patchset #2 (id:20001) manually as 42157 (presubmit successful).