add cachelib multi-tier example

[raema]

Starting this pull request as WIP for adding multi-tier cachelib example

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This change is 

[lukaszstolarczuk]

Reviewed 4 of 5 files at r1, all commit messages.
Reviewable status: 4 of 5 files reviewed, 1 unresolved discussion (waiting on @lukaszstolarczuk and @raema)

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examples/M/README.txt line 5 at r1 (raw file):

This example is intended for C++ programmers.

This is example consists of these files:

drop is here

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examples/M/README.txt line 7 at r1 (raw file):

This is example consists of these files:

main.cpp   -- ecample creating a mult-tier cache using pmem

FYI, misspell - ecample

[raema]

Thanks @lukaszstolarczuk I fixed these typos

[lukaszstolarczuk]

Reviewed 1 of 1 files at r2.
Reviewable status: 4 of 5 files reviewed, all discussions resolved (waiting on @lukaszstolarczuk)