The OmniSharp-roslyn loading issues appear to stem from situations where
multiple buffer-requests are sent before the project is fully
initialized. This results in duplicate "miscellaneous" files, which are
not correctly promoted to "project" files.

Instead, this commit introduces a buffer "initialization" concept, where
an `/updatebuffer` request is always sent as the first request for a
buffer, and no further requests are sent for that buffer until a
response is received (indicating that the server has registered the
file, and there will be no duplicates created).

Highlighting can work well as soon as the buffer is initialised (well
before the project is loaded). However, some aspects of highlighting are
project-dependent (e.g. pre-processors depending on project
configuration), so the _correct_ highlighting will only be applied after
the project has loaded completely.