Refactor code fore blockchain network management

[clangenb]

I did this as a preparation for task for #1603 in order to simplify management of multiple endpoints per network.

Changes

• Use an enum for the different networks instead of a json map containing network specific (nullable - why??) fields. This fixes many nullcheck !/? operators in the code base.
• Remove some outdated fields and computed of the settings store.
• We only cache the network-id instead of the whole network info type. The network info is static, no need to cache it. It also makes it hard in case the data structure (like adding multple endpoints per network ;)) changes.

Testing:

• The integration tests passed locally.