Transaction is broken

[tyru]

• Currently transaction is managed by file-based ($VOLTPATH/trx.lock file)
• In jsondsl branch (JSON DSL #236), transaction is changed by directory-based ($VOLTPATH/trx/{id}/...) because it's a simple and safe way to limit volt files accesses exclusively across many platforms
• After that, parallel tests were failed
  • Current exclusive lock implementation does not work completely
• jsondsl branch is stale, so let's re-implement the directory-based transaction from current devel branch

[tyru]

#296

1. Empty lock directory is created $VOLTPATH/trx/lock/
2. Delete the directory after transaction (something volt modifies files) ends