INT-3510: Fixed 'missing INT_MESSAGE_SEQ' error

JIRA: https://jira.spring.io/browse/INT-3510

There has been an error in schema creation scipt for message store on postgresql database:

The statement for creating sequence 'INT_MESSAGE_SEQ' was executed AFTER creating the table that actually uses this sequence
which resulted in sequence not found error.

Further problem was that the the corresponding index 'MSG_INDEX_DATE_IDX' in creation script does not match the index name
'INT_CHANNEL_MSG_DATE_IDX' in drop script. Therfore an error occurred when executing drop schema script for postgresql.

Fixes:

1.Moved creation of sequence 'INT_MESSAGE_SEQ' to first line to be executed before creating the corresponding table
2.Renamed index 'MSG_INDEX_DATE_IDX' to 'INT_CHANNEL_MSG_DATE_IDX' in create script to make drop script work again
On branch INT-3510

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