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.
Merge remote-tracking branch 'antirez/2.6' into 2.6