fix: index assets table on urn column

[sudo-suhas]

• Drop unique index on (urn, type, service) concurrently. This ensures
  that we don't take an exclusive lock on the table which would lock out
  selects, inserts, updates and deletes on the index's table.
• Create a unique index on urn column in the assets table.

Related to #162.

[StewartJingga]

@sudo-suhas I might've missed something here, but what is the reason for splitting the migration? can they be in one use case?

[sudo-suhas]

Already included the reason as a comment in one of the migration files:

-- By default, golang-migrate wraps multiple SQL statements in a transaction.
-- Dropping index concurrently is not allowed in a transaction. So the drop
-- statement needs to be the only statement in the migration.

Command output for failed migrate run:

➜ go run main.go migrate
Preparing migration...
INFO[0000] compass is migrating                          version=
INFO[0000] Migrating Postgres...
INFO[0000] Initiating Postgres client...
Usage:  compass migrate [flags]
problem with migration migration failed: migration failed: DROP INDEX CONCURRENTLY cannot run inside a transaction block in line 0: DROP INDEX CONCURRENTLY IF EXISTS assets_idx_urn_type_service;

CREATE UNIQUE INDEX assets_idx ON assets(urn);
 (details: pq: DROP INDEX CONCURRENTLY cannot run inside a transaction block)
exit status 1

[coveralls]

Pull Request Test Coverage Report for Build 2865844096

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.74%

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Totals
Change from base Build 2794118351:	0.0%
Covered Lines:	4726
Relevant Lines:	5512

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