Block "begin", "commit", "rollback" in lua

[dorinhogea]

"begin","rollback", "commit" and other non sqlite requests should fail to execute in sqlite.

Steps to reproduce the behavior:
cdb2sql $SPO - <<'EOF'
create procedure rcd version 'v0' {
local function main()
local _,rc0 = db:exec("begin")
local _,rc2 = db:exec("insert into t values (2)")
local __,rc3 = db:exec("rollback")
print("rc0 = "..rc0)
print("rc2 = "..rc2)
print("rc3 = "..rc3)
end
}$$
put default procedure rcd 'v0'
exec procedure rcd()
EOF

Server Output:
rc0 = 0
rc2 = 0
rc3 = 0

The rc0 and rc3 should be set to a proper error. Currently all exec-s succeed and the row is inserted.

[akshatsikarwar]

Additionally, executing dbstmt returned by these calls leaks a sosql session which fails an assert in subsequent runs. Minimal example:

local function main()
    db:exec("begin"):exec()
end