perf_dump detaches current memory segment, creates new [JIRA: RIAK-2104]

[matthewvon]

perf_dump is supposed to perform only read operations against the shared memory segment. However, it calls leveldb::Env::Default() which also accesses the shared memory segment and will resize it if necessary. This is bad behavior. Env object is only used for its NowTime(). Best bet is to likely copy that routine to perf_dump.cc, or create static equivalent that perf_dump can call (and is also called by env_posix's NowTime() routine).

[matthewvon]

mv-expiry branch adds leveldb::port::NowUint64() clock as alternative to Env::NowMicros(). perf_dump.cc updated to use new API in that branch.