tally stats for backend clients

[zachcheu]

Allow for backend clients to emit stats

Depends on #358

ok      github.com/uber/kraken/lib/backend      1.409s  coverage: 60.0% of statements
?       github.com/uber/kraken/lib/backend/backenderrors        [no test files]
ok      github.com/uber/kraken/lib/backend/gcsbackend   2.562s  coverage: 55.6% of statements
ok      github.com/uber/kraken/lib/backend/hdfsbackend  2.349s  coverage: 81.7% of statements
ok      github.com/uber/kraken/lib/backend/hdfsbackend/webhdfs  2.593s  coverage: 80.1% of statements
ok      github.com/uber/kraken/lib/backend/httpbackend  2.221s  coverage: 74.1% of statements
ok      github.com/uber/kraken/lib/backend/namepath     0.453s  coverage: 84.2% of statements
ok      github.com/uber/kraken/lib/backend/registrybackend      2.682s  coverage: 71.7% of statements
?       github.com/uber/kraken/lib/backend/registrybackend/security     [no test files]
ok      github.com/uber/kraken/lib/backend/s3backend    2.881s  coverage: 75.6% of statements
ok      github.com/uber/kraken/lib/backend/shadowbackend        3.485s  coverage: 79.8% of statements
ok      github.com/uber/kraken/lib/backend/sqlbackend   3.125s  coverage: 83.7% of statements
ok      github.com/uber/kraken/lib/backend/sqlbackend/benchmark 3.676s  coverage: [no statements] [no tests to run]
ok      github.com/uber/kraken/lib/backend/testfs       2.604s  coverage: 75.4% of statements