SL-11216: Introduce LLVersionInfo::getReleaseNotes() method.

The default string returned by getReleaseNotes() is empty. It must be set by
posting the relevant release-notes URL string to a new LLEventMailDrop
instance named "relnotes".

Add unique_ptr<LLEventMailDrop> and unique_ptr<LLStoreListener<std::string>>
to LLVersionInfo -- using unique_ptr to leave those classes opaque to
header-file consumers. Introduce an out-of-line destructor to handle the
unique_ptr<opaque> idiom.

Initialize the LLEventMailDrop with the desired name; initialize the
LLStoreListener with that LLEventMailDrop and the data member returned by
getReleaseNotes().