Added more info about 32 bit builds into README.
in 32bit machine, popcount don't work with a input string length up to 512 MB,
bitcount commant may return negtive integer with string length more than 256 MB
Test: various issues with the replication-4.tcl test fixed.

The test actually worked, but vars for master and slave were inverted
and sometimes used incorrectly.
Test: check that replication partial sync works if we break the link.

The test checks both successful syncs and unsuccessful ones by changing
the backlog size.
Version bumped to 2.7.1
Test: more PSYNC tests (backlog TTL).