Make --build-type a visible configure option.
Use CAF 0.11.0 with VAST.
Start working on tracker-based architecture.
Expose underlying vector.
Add new STL-compliant erase() overload.
Add new utility to retrieve hostname.
Overhaul node management.

The biggest change in this commit concerns the role of TRACKER, which has the
role to monitor all components in VAST deployment.

Other internal changes:

  - All actors are now mixins with pluggable *components*, which are just
    composable (but not associative) stateful message handlers. Each component
    has hooks for down/exit messages and they get executed if one chooses to
    override them.

    One particular component is "flow_controlled," which injects message
    handlers for flow control. Any flow_controlled actor understands the three
    message types announce, overload, and underload. For effective flow
    control, users need to establish a DAG of actors so that actors can signal
    their overload status upstream to the source.

    Another component is "sentinel," which just logs unexpected messages by
    appending an others() handler at the bottom.

  - Furthermore, schema resolving now occurs at QUERY and no longer at SEARCH
    because each arriving event has its own optimized AST for candidate
    checking.

  - New actors "load_balancer" and "replicator" which perform round-robin
    forwarding or 1-N message replication.

  - Prepared logging architecture for new simpler macros (upcoming).
Show default values in usage.
Fix linker error when only including trial<T>.