In a situation when we have lots of buckets the bootup time
might have slowed down a bit but during this situation the
servers quickly going up and down would be an in-transit state.
Certain calls which do not use quorum like `readXLMetaStat`
might return an error saying `errDiskNotFound` this is returned
in place of expected `errFileNotFound` which leads to an issue
where server doesn't start.
To avoid this situation we need to ignore them as safe values
to be ignored, for the most part these are network related errors.
Fixes#3275
- abstract out instrumentation information.
- use separate lockInstance type that encapsulates the nsMutex, volume,
path and opsID as the frontend or top-level lock object.
In a multipart upload scenario disks going down and coming backup
can lead to certain parts missing on the disk/server which was
going down. This is a valid case since these blocks can be
missing and should be healed through heal operation. But we are
not supposed to fail prematurely since we have enough data on
the other disks as well within read-quorum.
This fix relaxes previous assumption, fixes a major corruption
issue reproduced by @vadmeste.
Fixes#2976
