Fix RNG seeding

During a load test with high arrival rates, I noticed a lot of collisions of generated random strings. The collision rate was quite **reproducible at about 2%** (in a real world scenario).

The pull requests adds the tsung user id to the seed initialization used in `ts_client:init/1`.

Additionally, providing a fixed seed via `<option name="seed" value="4223"/>` also causes collisions across clients. In fact the collision rate was significantly higher (see below).

In my opinion both issues are severe, since you have to rely on the fact, that the randomness is strong enough. Collisions rates should be very close to 0, while all users have to be aware, that tsung does not provide crypto-grade randomness.

The following configuration can be used to reproduce the issue and validate the changes. Using this configuration, you'll see collision rates between **7%** (no fixed seed) and **98%** (fixed seed), using 924b5170533a09e7f404b3875b85129104e9312b.

## Test Configuration

```xml
<?xml version="1.0"?>
<!DOCTYPE tsung SYSTEM "/Users/basti/Documents/src/tsung/tsung-1.0.dtd">
<tsung loglevel="warning" version="1.0" dumptraffic="protocol">

  <clients>
    <client host="localhost" use_controller_vm="true"/>
  </clients>

  <servers>
    <server host="localhost" port="3000" type="tcp"></server>
  </servers>

  <load>
    <arrivalphase phase="1" duration="1000" unit="second">
      <users maxnumber="100000" arrivalrate="20000" unit="second"></users>
    </arrivalphase>
  </load>

  <options>
    <option name="seed" value="423128390213781273891273"/>
  </options>

  <sessions>
    <session name="randomness" probability="100" type="ts_http">
      <setdynvars sourcetype="eval"
                  code="fun(_) ->
                            Var = ts_utils:randombinstr(10),
                            io:format(&quot;~s~n&quot;, [binary_to_list(Var)])
                        end.">
      </setdynvars>
    </session>
  </sessions>
</tsung>
```

## Reproduce and Count Duplicates/Collisions

```
$ /tmp/tsung/bin/tsung -f ./randomness_test.xml -l ./logs start >| test_seed.out
$ tail -n +3 test_seed.out | sort | uniq | wc -l ; tail -n +3 test_seed.out | wc -l
```

For the current master and a fixed seed, this gives me `1152` and `100000`, which means **98.848% of all generated strings are duplicated values!** Without a fixed seed, `91409` and `100000`, which correlates to about 8.59% duplicates.

When this pull request is applied, you'll see **0 duplicates**.