Channel List API endpoint

[elpiel]

Implement the Channel List API Endpoint: https://github.com/AdExNetwork/adex-validator-stack-js/blob/master/routes/channel.js#L84

Current status: Some Channel data is returned. TODOs:

• [x] ~Hydrate the rest of the information for the channel, i.e. ChannelSpecs and the Validators of the ChannelSpec.~ As we are skipping the PostgresRepository implementation and working with the MemoryRepository we don't need this for now.
• [x] Add page query parameter #21
• [x] Add validator query parameter for filtering channels that contain a specific validator #21
• [x] Fetch only channels that have validUntil > now #20
• [x] Define a "CHANNELS_~FIND~LIST_LIMIT" for limit of channels per page (application configuration) #31
• [x] channel_list Handler limit per page to which we can pass the application configuration value #21
• [x] Filter options in ChannelRepository::list for page, limit & valid_until_ge #20
• [x] Filter option in ChannelRepository::list for validator #22
• [x] Implement the fromSql & toSql for BigNum ( I've opened an Issue in num-biging if they are able to help me to implement the BigUint for postgres and how to represent it in the database, for now it will remain ~u64~ String)

[elpiel]

Implement the fromSql & toSql for BigNum is Done, I've actually used a String representation which should be enough. Later on we can see for a better way of storing it in the Database and implement the ToSql and FromSql traits.

FromSql with String was implemented in https://github.com/AdExNetwork/adex-validator-stack-rust/commit/8f4a9f28598e4b58c7cbb46b2d4b57c310511b5f ToSql with String was implemented in PR #19