fix caching during bvfs_update

[arogge]

This patch replaces the complicated pathid_cache based on htable with a
much simpler one based on std::unordered_set. It also changes the
interface to use uint64_t instead of char*.

This addresses a problem where htable broke when it grew, because the key-memory had been reused.

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[arogge]

Just for the record: I shot myself in the foot again with the compiler not being able to detect broken format-strings on Mmsg(). We really need to get that fixed!

[pstorz]

Looks good!