There is a need to add session token to `eacl.Table` structure. To do this,
we need to replace `token.SessionToken` type to another package since `eacl`
package imports `token` one (potential cross-import).
Create `pkg/session` package and replace session token implementation to it.
Related API in `container` package is deprecated from now.
Additionally implement test generator of random session tokens.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
