Add key uniqueness validation

[irondnb]

• validation for uniqueness of key-target in Subscription model

• move create_subbscription method to build_subscription, so we can check for errors before save without rising exception

• left create_subbscription method for test purposes

• refactor some test: lazy loading objects

[coveralls]

Coverage decreased (-0.01%) to 99.989% when pulling 7cce4cb on irondnb:add-key-uniqueness-validation into 9db86fe on simukappu:development.

[coveralls]

Coverage decreased (-0.1%) to 99.882% when pulling fad3bc6 on irondnb:add-key-uniqueness-validation into 9559d13 on simukappu:development.

[coveralls]

Coverage decreased (-0.1%) to 99.882% when pulling fad3bc6 on irondnb:add-key-uniqueness-validation into 9559d13 on simukappu:development.

[coveralls]

Coverage decreased (-0.1%) to 99.882% when pulling fad3bc6 on irondnb:add-key-uniqueness-validation into 9559d13 on simukappu:development.

[coveralls]

Coverage decreased (-0.1%) to 99.882% when pulling fad3bc6 on irondnb:add-key-uniqueness-validation into 9559d13 on simukappu:development.

[irondnb]

working on tests

Edit: Since db was cleared only once, there will be duplicates during test.
Temporarly i'm clearing it before each test which result in extended execution time.
Sadly, there are still some mysql test that failing

[irondnb]

@simukappu I've tried, but it end up in more commits 🤕
I will just create another PR with new branch.

Note: I've drop support for dynamodb for now and will fix it in another PR

[simukappu]

I've rewrite your contribution and pushed as this commit: 3b46954
Also added dynamoid uniqueness validation.

Thank you for your contribution!