Inconsistent CMAKE_BUILD_TYPE

Currently, CMakeLists.txt unconditionally adds -g to the compiler flags, resulting in all builds containing debug symbols:

https://github.com/Icinga/icinga2/blob/4b754637f7ac06328d101c72fd03e897707fd352/CMakeLists.txt#L251-L265

I find this surprising as this makes the existence of the two CMAKE_BUILD_TYPES of Release and RelWithDebInfo a bit pointless.

Relevant commits/issues from the past touching this (unfortunately with little reasoning why they did it this way):

• https://github.com/Icinga/icinga2/commit/e5fba1d881c7de0b16ddf8fd662edb9699cc8b44 + https://github.com/Icinga/icinga2/issues/958
• https://github.com/Icinga/icinga2/commit/b6220c6ecfd3a64bf73ae5de8ed62f257867b5c0 + https://github.com/Icinga/icinga2/issues/998

Tasks

• Figure out if using RelWithDebInfo instead of Release makes more difference than passing -g additionally.
• Prepare PR that does not unconditionally add -g to compiler flags.
• Test that debug symbol packages are still built properly.
• Make sure docker builds use the Release type and don't strip the binaries themselves (as introduced by https://github.com/Icinga/docker-icinga2/pull/85)

Start with CMakeLists.txt around lines 251-265 and compare the Release and RelWithDebInfo configurations. Check the resulting compiler flags, debug symbol packages, and the docker build behavior referenced in the issue. Done means -g is not added unconditionally, symbol packages still build correctly, and docker builds use Release without stripping binaries.