Memory optimization when using aws s3 (Try 2)

[ebressler]

The changes here make it so that there are no longer references being
held to the s3 objects allocated during XMLToStruct in the s3 libraries.

In my testing, when doing a listObjectsV2 against ~383,000 objects, this
change saves about 1/2 of the memory uses. (1gb of use without these
changes and ~500mb with them)

This is an updated way of doing this. I somehow closed out #547 and can't reopen the PR.

[ebressler]

@kahing, I found a better way of doing the fix that should work across all backends. The other PR I accidentally closed when redoing my branch.

before:

File: __debug_bin
Type: inuse_space
Time: Aug 24, 2020 at 10:39am (EDT)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 416.55MB, 98.76% of 421.76MB total
Dropped 29 nodes (cum <= 2.11MB)
Showing top 10 nodes out of 25
      flat  flat%   sum%        cum   cum%
  237.03MB 56.20% 56.20%   320.04MB 75.88%  github.com/kahing/goofys/vendor/github.com/aws/aws-sdk-go/private/protocol/xml/xmlutil.XMLToStruct
   72.50MB 17.19% 73.39%    72.50MB 17.19%  github.com/kahing/goofys/vendor/github.com/aws/aws-sdk-go/private/protocol/xml/xmlutil.(*XMLNode).findNamespaces
   48.51MB 11.50% 84.89%    48.51MB 11.50%  github.com/kahing/goofys/internal.(*Inode).SetFromBlobItem
   37.50MB  8.89% 93.79%    37.50MB  8.89%  github.com/kahing/goofys/internal.NewInode
       6MB  1.42% 95.21%        6MB  1.42%  encoding/xml.(*Decoder).name
    5.78MB  1.37% 96.58%        8MB  1.90%  github.com/kahing/goofys/internal.(*Goofys).insertInode
       3MB  0.71% 97.29%        3MB  0.71%  encoding/xml.makeCopy
       3MB  0.71% 98.00%    96.01MB 22.77%  github.com/kahing/goofys/internal.(*DirHandle).ReadDir
    2.21MB  0.52% 98.53%     2.21MB  0.52%  github.com/kahing/goofys/internal.(*Inode).insertChildUnlocked
       1MB  0.24% 98.76%     7.50MB  1.78%  encoding/xml.(*Decoder).Token

After:

File: __debug_bin
Type: inuse_space
Time: Aug 24, 2020 at 10:45am (EDT)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 287.37MB, 97.37% of 295.12MB total
Dropped 76 nodes (cum <= 1.48MB)
Showing top 10 nodes out of 26
      flat  flat%   sum%        cum   cum%
  143.54MB 48.64% 48.64%   149.54MB 50.67%  github.com/kahing/goofys/internal.(*Inode).SetFromBlobItem
   71.51MB 24.23% 72.87%    71.51MB 24.23%  github.com/kahing/goofys/internal.NewInode
   45.50MB 15.42% 88.29%    50.04MB 16.96%  github.com/kahing/goofys/vendor/github.com/aws/aws-sdk-go/private/protocol/xml/xmlutil.XMLToStruct
    9.56MB  3.24% 91.53%    14.81MB  5.02%  github.com/kahing/goofys/internal.(*Goofys).insertInode
       6MB  2.03% 93.56%        6MB  2.03%  github.com/kahing/goofys/internal.deepCopyBlobItemOputput
    4.50MB  1.52% 95.09%   240.36MB 81.45%  github.com/kahing/goofys/internal.(*DirHandle).ReadDir
    3.25MB  1.10% 96.19%     3.25MB  1.10%  github.com/kahing/goofys/internal.(*Inode).insertChildUnlocked
    2.50MB  0.85% 97.03%     3.04MB  1.03%  encoding/xml.(*Decoder).rawToken
       1MB  0.34% 97.37%     4.04MB  1.37%  encoding/xml.(*Decoder).Token
         0     0% 97.37%   240.36MB 81.45%  github.com/kahing/goofys/api/common.FusePanicLogger.ReadDir

[ebressler]

@kahing , just checking in to make sure you have not missed this update.

[ebressler]

@kahing anything you need from me on this?

[ebressler]

@kahing , is there anything else that I need to do here or is this good?

[kahing]

b12cef3