Fix enum NumConnections shadowing warning

Summary:
Reduce the scope of the NumConnections enum in clientModel.cpp by
making it part of the ClientModel class.

This is an alternative to D4681.

Test Plan:

ninja check

Check the -Wshadow warning no longer exist.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4721