Fixes for checkpoint/restart workflow

[ChristopherHogan]

• Add HERMES_CLIENT environment variable so that a single client app can connect to an existing Hermes daemon. Fixes Need a better way to distinguish daemon vs. client in adapters #206.
• Populate AdapterStat::st_size when an adapter client detects that a Bucket already exists in Hermes.
• Don't overwrite GLOG_minloglevel if it's already set.
• Fix some integer overflows in buffer_pool.cc.
• Return an error Status when CalculatePlacement can't find any valid Targets.
• Take the bucket mutex when retrieving a Bucket's BlobIDs.

[coveralls]

Coverage increased (+0.05%) to 80.852% when pulling 8c292d3 on chogan/checkpoint_restart into d581dab on master.