apache / bookkeeper / 324

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Allow to configure sticky reads

### Motivation

Currently the BK client is issuing the read requests in round-robin fashion across all the bookies in the write set.

One issue with this approach is that it's not taking full advantage of the read-ahead cache, either explicit (like in `DbLedgerStorage`) or implicit (by reading data through Linux page cache which will do some prefetching).

With `e=2`, `w=2`, when we read `e-0` from `bookie-1` and `e-1` from `bookie-2`, we fail to take advantage of the fact that `bookie-1` will have already `e-1` in memory.

Effectively with `e-2`, `w-2` the disk read IO will be doubled, compared to the amount of data served to BK clients. The larger the quorum, the bigger will be overhead (eg: `e=5`, `w=5` will lead to 5x reads from disk).

### Changes

Added a BK client flag for "sticky reads". When reading from a ledger that has `E=W` (every bookie has all the entries), the sticky reads will direct all read request to 1 single bookie in the ensemble.

Reviewers: Enrico Olivelli <eolivelli@gmail.com>, Sijie Guo <sijie@apache.org>

This closes #1808 from merlimat/single-bookie-per-ledger-read-master

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