The describe requests were launching a storm of RLocks which weren't
safe in the presence of goroutines trying to acquire write locks.
Instead, make the corpus locking the responsibility of the caller and
add Lock/Unlock/RLock/RUnlock methods to the index and move locking up
a level.
This also adds a fair bit of context.Context plumbing which was used
in earlier debugging.
Fixescamlistore/camlistore#709
Change-Id: I8d7254d1e1da541f8c080d62f5408aac807fd3b1
Most of it replaced with vendor/go4.org/types and
vendor/go4.org/readerutil
u32 went where needed in pkg/blobserver/*
invertedBool went in pkg/types/serverconfig
atomics64 went in pkg/fs
Change-Id: I230426cda35be4b45ed67e869f14e6fdae89be22
Brad Fitzpatrick