[CMAKE] Make RelWithDebInfo the default CMake configuration

Summary:
This set RelWithDebInfo as the default build type for CMake. This
makes the default optimization flags identical to those from
configure.

Depends on D2651

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja ..
ninja -v

Check that the -g -O2 flags are set in the compile commands for both
CC and CXX compile lines.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2643