refactor README

Signed-off-by: Andy Green <andy.green@linaro.org>
introduce without extensions

The new --without-extensions config flag completely removes all code
and data related to extensions from the build throughout the library
when given.

Signed-off-by: Andy Green <andy.green@linaro.org>
zlib not needed if no extensions

Signed-off-by: Andy Green <andy.green@linaro.org>
log major dynamic allocation info

Signed-off-by: Andy Green <andy.green@linaro.org>
document memory efficiency

This puts some numbers of library size with the various --without
and --disable options and about dynamic memory allocation
performance

Signed-off-by: Andy Green <andy.green@linaro.org>
align test server extpoll with library dynamic approach

Signed-off-by: Edwin van den Oetelaar <oetelaar.automatisering@gmail.com>