Cache WASM_LLVM_CONFIG value

WASM_LLVM_CONFIG will now be a properly cached variable, settable with
cmake -DWASM_LLVM_CONFIG=/path/to/llvm-config and persisting across runs
of cmake and editable with ccmake|cmake-gui.  If the value was not
specified on the command line or retrieved from the cache, it will
resort to the environment variable.

Added gitlab ci docker building

Added argument propagation with docker

/programs/snapshot submodule

Benchmark

Log 244

update max block size, fix performance of chain_controller::head_block_producer, and increase batch sizes for benchmark

Added skip-transaction-signatures requirement

A new test for launching eosd, giving it time to produce blocks, kill…

Add a single threaded scheduler and make it the default

Add doxygen guidelines