Accumulator refactor:

Stores a single copy of each request from different sources.
Firewalls identical requests.
Implements time based eviction policy.
Tests.
Merge branch 'next' into MAID-563_accumulator_update

Conflicts:
	src/maidsafe/vault/tests/accumulator_test.cc
Linux fix, added request lifetime to Parameters.
Linux fix.
Minor change.
Merge remote-tracking branch 'upstream/next' into MAID-563_accumulator_update
Style fix, tidyup and new test.
Further tidyup.
Updated in line with comments.
Merge pull request #92 from BSMaidSafe/MAID-563_accumulator_update

Accumulator refactor: