Add the concept of saga completion.

[jmalloc]

Fixes #16

Saga's can now be "completable". When an instance is completed, the saga persistence and mapping systems have an opportunity to remove persisted data. The exact behaviour is implementation depending, with the following behaviour implemented currently:

When a saga instance is completed, the:

• eventsourcing persistence layer deletes all snapshots of the instance
• crud persistence layer hard-deletes the instance
• keyset mapping strategy hard-deletes message->instance mapping data

[codecov-io]

Codecov Report

Merging #109 into master will decrease coverage by 0.95%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
- Coverage   30.24%   29.28%   -0.96%     
==========================================
  Files          65       65              
  Lines        2169     2240      +71     
==========================================
  Hits          656      656              
- Misses       1503     1574      +71     
  Partials       10       10

Impacted Files	Coverage Δ
src/ax/aggregate/saga.go	0% <ø> (ø)	:arrow_up:
.../ax/saga/persistence/eventsourcing/messagestore.go	0% <ø> (ø)	:arrow_up:
src/ax/saga/persistence/eventsourcing/persister.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/handler.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/persistence/crud/persister.go	0% <0%> (ø)	:arrow_up:
src/axmysql/saga/snapshot.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/mapping/direct/mapper.go	0% <0%> (ø)	:arrow_up:
src/axmysql/saga/keyset.go	0% <0%> (ø)	:arrow_up:
src/ax/saga/behavior.go	0% <0%> (ø)	:arrow_up:
src/axmysql/saga/crud.go	0% <0%> (ø)	:arrow_up:
... and 1 more

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