feat(sdk): reuse dapi connections

[lklimek]

Issue being fixed or feature implemented

We establish new connection to DAPI for every request. This has significant impact on performance.

What was done?

Implemented connection pool that allows reuse of clients.

At his point, the connection pool uses LRU (least-recently used) algorithm to evict unused peers.

How Has This Been Tested?

1. Run rs-sdk tests in network mode.
2. Tested on testnet using load_test from rs-platform-explorer.

Breaking Changes

None

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have added "!" to the title and described breaking changes in the corresponding section if my code contains any
• I have made corresponding changes to the documentation if needed

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone