Fix running of MongoChannelStoreWriteTest with other test cases

This test was failing because the test mongo and channel stores
were static final variables and initialised at class loading time.
Therefore the initialisation of database performed in
MongoSequentialIdGenerator was being lost by other classes
subsequently getting a test mongo, which drops the db, ahead of
the tests in MongoChannelStoreWriteTest being run.
Fix MongoScheduleStoreTest

The signature of MongoScheduleStore.schedule() has changed and now takes
an Optional<ApplicationConfiguration> rather than an
ApplicationConfiguration which can be null. The test was not updated at
that time, so was causing an NPE.
Merge pull request #74 from atlasapi/fix-tests

Fix MongoChannelStoreWriteTest and MongoScheduleStoreTest