Bump OpenDHT minimum version and add DHT cache mechanism - #32

This pull request introduces disk caching for the DHT identity and node state in dpaste, significantly improving startup performance by enabling warm starts instead of cold-bootstrapping the DHT on every run. It also updates the required OpenDHT version from 1.2.0 to 3.0.0 across build scripts and documentation.

Caching improvements:

• Added persistent caching of the DHT identity and node state in the user's cache directory (defaulting to ${XDG_CACHE_HOME}/dpaste), with support for overriding the location via the DPASTE_CACHE_DIR environment variable. The cache is loaded on startup and refreshed on shutdown, resulting in much faster subsequent startups. (src/node.cpp, src/node.h) [1] [2] [3]
• Implemented logic to safely generate, cache, and load the DHT identity, handling concurrent runs and corrupt cache files gracefully. (src/node.cpp, src/node.h) [1] [2]

Documentation updates:

• Added a new "Caching" section to the README.md to explain the new caching behavior and environment variable.

Dependency updates:

• Updated the required OpenDHT version to 3.0.0 in CMakeLists.txt, configure.ac, and README.md to reflect compatibility with the new caching features and API. [1] [2] [3]