block: eliminate thread in generic block_component

Instead of using an additional thread in each Session_component of a
block server, which uses the generic block component and driver classes,
replace it with signal dispatchers.

Ref #113
block: extend interface of generic driver class

To support read-only devices like CDROMs, or rom_blk the supported
block operations have to be stated by specific the device driver.

Ref #966
rom_loopdev: use generic block component/driver

Ref #966
rom_loopdev: rename to rom_blk to be consistent

Ref #966
atapi_drv: use generic block component/driver

Ref #966
fb_block_adapter: use generic block component/driver

Ref #966
http_block: use generic block component/driver

Ref #966
http_block: rename to http_blk to be consistent

Ref #966
rom_blk test: abort if test fails

Instead of printing the success message at the end of the test,
abort as soon as two blocks differ.

Ref #966
http_blk: use libc_lwip plugin, not lwip directly

Ref #966
http_blk: add run script for automated tests

Ref #966