serverinit: add support for memory-only indexer. Enable in devcam.

To avoid confusion, the previously named "memoryIndex" configuration
parameter, which actually meant to slurp the index into the memory
corpus, has been renamed to "copyIndexToMemory". "memoryIndex" is now
the parameter to enable a memory-only indexer.

http://camlistore.org/issue/416

Change-Id: I7026e5463fda3a1887a88e0b07d4e3431c60c8b7
blobserver: add memory storage, mainly for tests.

Mostly code moved from pkg/test/fetcher.go

Also got rid of blob.MemoryStorage

http://camlistore.org/issue/416

Change-Id: I8d17b80942e2fde1c869f71738bfb8a67ed8ab31
Merge "blobserver: add memory storage, mainly for tests."
Merge "serverinit: add support for memory-only indexer. Enable in devcam."
schema: fix test I broken when removing GetChunkOffsets

Rewrite in terms of ForeachChunk instead.

Change-Id: Id018420c47ffe116246c92b06c61a9dc9bbbee62