add multiple probability distribution options to dw_client

[tomcucinotta]

The client currently only supports specification of fixed or exponentially distributed processing times, inter-arrival times, and send or reply packet sizes. However, there is very little ability to specify different combinations of these parameters at the moment, especially if using chains of multiple commands to be executed on different hosts using the FORWARD.
We should allow for a richer set of distributions, e.g.:

• uniformly distributed within a (min,max) range;
• exp-distributed with explicit (min,max) caps;
• normally distributed with specified average and standard deviation;
• Gamma distribution with parameters k and theta;
• generic PMF distribution specified as vector of equally-sized bins;
• generic distribution specified as samples of the CDF;
• file-based samples, i.e., values read from client-side file containing experimental samples that are simply replayed.

[tomcucinotta]

With commit c13e598, the client is allowed to specify for inter-arrival times, packet-size, response-packet size, computation times, either:

• a numeric value, just entering the value on the command-line
• a uniform distribution, through the syntax 'unif(min,max)', or the syntax 'unif(max)' where min=0 is taken implicitly
• an exponential distribution, through the syntax 'exp(avg)'

for example, for the -C option specifying computation times:

• -C 10000
• -C 'exp(10000)'
• -C 'unif(10000,20000)'
• -C 'unif(10000)', which is equivalent to -C 'unif(0,10000)'

The baseline code in distrib.c should allow easily to plug additional syntaxes for additional probability distributions.

May need some more testing, especially when used in conjunction with the rate control options (ramp and samples from file).

[tomcucinotta]

this is done, supporting all distributions identified above, except the random sampling from experimental PMF, which can actually be emulated (alongside other arbitrary distributions) creating a file with a trace of the desired distribution, then using the "file:" syntax.