common: Add WebCache

common: Add WebCache

A cache data structure for values that need to be retrieved form
outside systems (e.g Key Management Systems).

Features:
- Thread safe, optimized for concurrent lookups and cache hits
- Entry TTL expiration
- Cache replacement strategy tuned to "web" workloads (SIEVE)
- Performance Counters on hit, miss, expire, size, capacity, clears

Signed-off-by: Marcel Lauhoff <marcel.lauhoff@clyso.com>
On-behalf-of: SAP marcel.lauhoff@sap.com