txnengine, txnstorage: various changes (#4553)

txnengine: add Errors and extract errors in doTxnRequest
txnengine: add Errors.As; create default database; fix Execution.Resolve
txnengine: add MemHandler.mheap; fix MemHandler.HandleRead
txnengine: add TestTPCH
txnengine: add Tx.databaseName
txnengine: add file path param to Tx.Exec
txnengine: add typed errors
txnengine: fix Execution.Resolve
txnengine: fix Tx.Exec to execute all statements
txnengine: fix engineAttrToPlanColDef
txnengine: fix plan types gob registeration
txnengine: gob register plan types
txnengine: save database and table name in lowercase; set default schema name in Execution.Resolve
txnengine: set zap logger for NewTxnService
txnengine: trim comment lines in bvt files
txnstorage: add AttributeRow.Order
txnstorage: add hidden __version column
txnstorage: fix BatchIter
txnstorage: fix MemHandler.HandleRead
txnstorage: use req.TableID,DatabaseID in HandleDelTableDef,HandleGetRelations,HandleGetTableDefs,HandleGetTableDefs

Approved by: @lni