Replace Horizon calls with RPC

[aditya1702]

What

• Replace horizon calls for fetching account details with RPC.
• Use TSS to submit the channel account creation operations instead of Horizon.
• Remove all horizon related flags, config variables and code pieces.
• Update tests to account for the above new changes.

Why

Horizon is huge monolith that is very hard to maintain and this PR is part of org wide effort to move away from Horizon and use RPC instead which is very lightweight and easier to run than Horizon.

Known limitations

N/A

Issue that this PR addresses

Closes #75

Checklist

PR Structure

• It is not possible to break this PR down into smaller PRs.
• This PR does not mix refactoring changes with feature changes.
• This PR's title starts with name of package that is most changed in the PR, or all if the changes are broad or impact many packages.

Thoroughness

• This PR adds tests for the new functionality or fixes.
• All updated queries have been tested (refer to this check if the data set returned by the updated query is expected to be same as the original one).

Release

• This is not a breaking change.
• This is ready to be tested in development.
• The new functionality is gated with a feature flag if this is not ready for production.

[JakeUrban]

This looks mostly good! Just a couple minor comments and one more meaningful question.

[gouthamp-stellar]

One thing to note here is that if we are using RPC, we need to be sure that it is healthy (ie gethealth returns a positive result), which can take upto a few minutes if I'm not mistaken. So you may not be able to create all those channel accounts as soon as wallet backend starts up - you'd have to wait till RPC is ready to create those channel accounts right?

[gouthamp-stellar]

take a look at my comments