feat: add decoding engine

[baltzell]

In the past year, the decoder was sped up enough to be usable as the (single-threaded) CLARA I/O service DecoderReader. With current reconstruction speeds, that scales linearly up to about 32 threads, where it becomes I/O-bound by (single-threaded) decoding.

This pull request implements the decoder as a (multi-threaded) CLARA engine DecoderEngine, based on a pool of CLASDecoder objects, in lieu of a thread-safe decoder. To minimize initialization overhead, the underlying ConstantManager objects are shared by the whole pool, similar to ReconstructionEngine.

Here's the rough performance for a 24-thread job on clara2502 with etc/services/rgd-clarode.yml. Note, the 12-ms-per-event for the DECO engine suggests some thread contention, since it's few-ms when run single-threaded.

READER          10000 events    total time =     0.52 s    average event time =    0.05 ms
DECO            10000 events    total time =   129.11 s    average event time =   12.91 ms
DCDN            10000 events    total time =   417.51 s    average event time =   41.75 ms
MAGFIELDS       10000 events    total time =     0.17 s    average event time =    0.02 ms
FTCAL           10000 events    total time =     0.24 s    average event time =    0.02 ms
FTHODO          10000 events    total time =     0.18 s    average event time =    0.02 ms
FTTRK           10000 events    total time =     1.25 s    average event time =    0.12 ms
FTEB            10000 events    total time =     0.19 s    average event time =    0.02 ms
RASTER          10000 events    total time =     0.26 s    average event time =    0.03 ms
DCCR            10000 events    total time =   102.01 s    average event time =   10.20 ms
MLTD            10000 events    total time =   171.65 s    average event time =   17.17 ms
DCHAI           10000 events    total time =   440.76 s    average event time =   44.08 ms
FTOFHB          10000 events    total time =    34.57 s    average event time =    3.46 ms
EC              10000 events    total time =    17.54 s    average event time =    1.75 ms
CVTFP           10000 events    total time =   938.05 s    average event time =   93.80 ms
CTOF            10000 events    total time =    18.39 s    average event time =    1.84 ms
CND             10000 events    total time =     5.45 s    average event time =    0.54 ms
BAND            10000 events    total time =     1.07 s    average event time =    0.11 ms
HTCC            10000 events    total time =     0.74 s    average event time =    0.07 ms
LTCC            10000 events    total time =     0.58 s    average event time =    0.06 ms
EBHB            10000 events    total time =    10.60 s    average event time =    1.06 ms
DCTB            10000 events    total time =  1439.13 s    average event time =  143.91 ms
FMT             10000 events    total time =    43.14 s    average event time =    4.31 ms
CVTSP           10000 events    total time =   346.16 s    average event time =   34.62 ms
FTOFTB          10000 events    total time =    25.53 s    average event time =    2.55 ms
EBTB            10000 events    total time =     9.04 s    average event time =    0.90 ms
RICH            10000 events    total time =    65.09 s    average event time =    6.51 ms
RTPC            10000 events    total time =     0.34 s    average event time =    0.03 ms
VTX             10000 events    total time =   187.63 s    average event time =   18.76 ms
CALIB           10000 events    total time =     7.27 s    average event time =    0.73 ms
WRITER          10000 events    total time =     2.84 s    average event time =    0.28 ms
TOTAL           10000 events    total time =  4417.00 s    average event time =  441.70 ms