* remove old raft servers if they don't answer to pings for too long
add ping durations as options
rename ping fields
fix some todos
get masters through masterclient
raft remove server from leader
use raft servers to ping them
CheckMastersAlive for hashicorp raft only
* prepare blocking ping
* pass waitForReady as param
* pass waitForReady through all functions
* waitForReady works
* refactor
* remove unneeded params
* rollback unneeded changes
* fix
When multiple filer requests are in-flight and the current filer
disappears and a new one is selected by the first goroutine, then
there can be a lot of race conditions while retrieving the current
filer.
Therefore, load/save the current filer index atomically.
