Better comment for inplace_update_support

Summary: See https://github.com/facebook/rocksdb/issues/215

Test Plan: none

Reviewers: dhruba, sdong, ljin, yhchiang, nkg-

Reviewed By: nkg-

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20769
Fix ldb dump_manifest

Summary:
We now reads table properties in VersionSet::LogAndApply(), which requires options.db_paths to be set. But since ldb_cmd directly creates VersionSet without initialization db_paths, causing a seg fault. This patch fix it by initializing db_paths.

log_and_apply_bench still shows segfault, because table cache is nullptr in VersionSet created.

Test Plan: Run ldb dump_manifest which used to fail.

Reviewers: yhchiang, ljin, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20751