Fix coinbase caching

[sirlensalot]

Big improvement

benchmarking PactService/forkingBench
time                 506.3 ms   (486.9 ms .. 529.5 ms)
benchmarking PactService/block-new-valid[1]
time                 7.792 ms   (7.619 ms .. 7.953 ms)
benchmarking PactService/block-new-valid[10]
time                 52.66 ms   (50.48 ms .. 54.22 ms)
benchmarking PactService/block-new-valid[50]
time                 251.3 ms   (233.6 ms .. 268.1 ms)
benchmarking PactService/block-new-valid[100]
time                 483.7 ms   (459.9 ms .. 507.3 ms)
benchmarking PactService/block-new[1]
time                 3.420 ms   (3.371 ms .. 3.476 ms)
benchmarking PactService/block-new[10]
time                 26.44 ms   (26.19 ms .. 26.78 ms)
benchmarking PactService/block-new[50]
time                 128.4 ms   (124.7 ms .. 131.3 ms)
benchmarking PactService/block-new[100]
time                 254.0 ms   (239.1 ms .. 267.4 ms)

previously:

benchmarking PactService/forkingBench
time                 2.939 s    (2.712 s .. 3.074 s)
benchmarking PactService/block-new-valid[1]
time                 258.7 ms   (244.0 ms .. 299.3 ms)
benchmarking PactService/block-new-valid[10]
time                 278.8 ms   (275.3 ms .. 282.2 ms)
benchmarking PactService/block-new-valid[50]
time                 467.8 ms   (450.9 ms .. 489.9 ms)
benchmarking PactService/block-new-valid[100]
time                 725.9 ms   (707.2 ms .. 741.7 ms)
benchmarking PactService/block-new[1]
time                 119.9 ms   (117.2 ms .. 123.2 ms)
benchmarking PactService/block-new[10]
time                 139.3 ms   (138.0 ms .. 141.3 ms)
benchmarking PactService/block-new[50]
time                 243.9 ms   (235.3 ms .. 254.7 ms)
benchmarking PactService/block-new[100]
time                 370.7 ms   (357.6 ms .. 386.0 ms)