scylla-gdb.py: scylla memory: better summary of semaphore memory usage
scylla-gdb.py: scylla memory: better summary of semaphore memory usage

If available, use the recently added
`reader_concurrency_semaphore::_initial_resources` to calculate the
amount of memory used out of the initially configured amount. If not
available, the summary falls back to the previous mode of just printing
the remaining amount of memory.
Example:
Replica:
  Read Concurrency Semaphores:
    user sstable reads:       11/100,     263621214/     42949672 B, queued: 847
    streaming sstable reads:   0/ 10,             0/     42949672 B, queued: 0
    system sstable reads:      1/ 10,        251584/     42949672 B, queued: 0

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20200901091452.806419-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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