Add warning when `tcl` is not available to run tests

Use a safe iterator while saving the DB, since the getExpire() function will access the iterating dictionary.

Fix automatic rewrite starting too early

Slight adjustment in growth calculation.

Small fix in the installation instructions

echo with escape sequences isn't portable. Use printf instead.

Two fixes for replication: Slave performs the AOF rewrite at the right point. Non blocking connect also uses readable handler as with old Linux kernels like 2.6.18 on connection refused the writable even is not fired (kernel bug).