rpc: Default rpcthreads value too small(4)

[naftalimurgor]

Summary

Hello @wu-emma @van-orton

This issue aims at :-

• Increase the default number of RPC threads to 16 and remove outdated references to older Bitgesell Core versions.

• Contributors to feel free to flag any other inconsistencies you come across with the current defaults.

Motivation

This Issue is motivated by this Bitcoin Core issue/implementation: Default rpcthreads value (4) is too small