vinyl: refactor tx statistics

Remove box.info.vinyl().performance.{tx_rlb,tx_conflict}. Instead add
'tx' section to box.info.vinyl() with the following counters:

 - 'active': number of active transactions
 - 'commit': number of committed transactions
 - 'rollback': number of rolled back transactions
 - 'conflict': number of transactions aborted on conflict

Apart from improving vinyl statistics, this patch also pursues one more
goal: removing dependency of vy_tx and tx_manager on vy_env, which is
needed to move them to a separate source file.

Needed for #1662 #1906