feat: Merge local manifest with fetched remote manifest

feat: Merge local manifest with fetched remote manifest

With read-only secondary storage, it can happen that primary storage has
a manifest named M with a result entry R1, while secondary storage also
has a manifest M but with result R2. On a compilation that matches R2,
ccache will first succeed to look up M in primary storage, fail to find
R2 and then get M from secondary storage where R2 can be found. Since M
already exists locally, ccache will simply return the cache hit but not
store knowledge of R2 locally. On a rebuild of R2, ccache therefore
needs to fetch from secondary storage again.

The improvement brought by this commit is that ccache now merges the
manifests from primary and secondary storage and stores the merged
version in primary storage. In other words, ccache setups with read-only
secondary storage will be able to accumulate local header file
combinations and seamlessly combine them with changes from secondary
storage.

Closes #1049.