Replace mongoose with Boost ASIO and support keep alive connections

[edubart]

Implementation details:

• The client uses synchronous IO.
• In the client, special care is taken when connect/write/read calls are interrupted, so the client does not lose track of spawned remote machines, otherwise it could leave dead zombies when its process receives SIGTERM/SIGINT.
• The client persist connections with keep alive, so we speed up request throughput
• The client still use SO_LINGER option to minimize TIME_WAIT connections during fork
• Both client and server uses TCP_NODELAY to minimize connection latency
• The server uses asynchronous IO, so we can supporting connecting multiple clients in parallel and forking them.
• The server only shut downs when all asynchronous operations are either completed or cancelled. Meaning SIGTERM will not make it close right-away, it will only gracefully terminate when all clients sessions are gracefully closed by the client.
• In the server, special care is taken during the fork.
• Server uses SO_REUSEADDR flag
• I've opted to disable threads, EPOLL and EVENTFD, to force Boost ASIO use select() based implementation, so we have less chance of issues with fork().
• Boost ASIO was used for networking
• Boost Beast was used for parsing HTTP requests/responses
• There is no need to link any library, only header-only libraries from Boost were used.
• Nlohmann JSON library was bundled in third party
• make dep and make downloads were removed
• I did some stress tests on the changes, ran thousands of requests/forks per second, found no issue so far.
• All mongoose related code was removed.

This replaces:

• Use keep alive connection for JSONRPC client #236

This should close:

• Optimize jsonrpc request throughput #222
• make dep fails if third-party/downloads/ directory exists (even if it is empty) #157

[diegonehab]

This is looking good.