Don't allocate huge slices to merge pgids in freelist.write

Using a large (50gb) database with a read-write-delete heavy load,
nearly 100% of allocated space came from freelists.
1/3 came from freelist.release, 1/3 from freelist.write,
and 1/3 came from tx.allocate to make space for freelist.write.
In the case of freelist.write, the newly allocated giant slice gets
copied to the space prepared by tx.allocate and then discarded.

To avoid this, add func mergepgids that accepts a destination slice,
and use it in freelist.write.

This has a mild negative impact on the existing benchmarks,
but cuts allocated space in my real world db by over 30%.

name                      old time/op    new time/op    delta
_FreelistRelease10K-8       18.7µs ±10%    18.2µs ± 4%    ~             (p=0.548 n=5+5)
_FreelistRelease100K-8       233µs ± 5%     258µs ±20%    ~             (p=0.151 n=5+5)
_FreelistRelease1000K-8     3.34ms ± 8%    3.13ms ± 8%    ~             (p=0.151 n=5+5)
_FreelistRelease10000K-8    32.3ms ± 1%    32.2ms ± 7%    ~             (p=0.690 n=5+5)
DBBatchAutomatic-8          2.18ms ± 3%    2.19ms ± 4%    ~             (p=0.421 n=5+5)
DBBatchSingle-8              140ms ± 6%     140ms ± 4%    ~             (p=0.841 n=5+5)
DBBatchManual10x100-8       4.41ms ± 2%    4.37ms ± 3%    ~             (p=0.548 n=5+5)

name                      old alloc/op   new alloc/op   delta
_FreelistRelease10K-8       82.5kB ± 0%    82.5kB ± 0%    ~     (all samples are equal)
_FreelistRelease100K-8       805kB ± 0%     805kB ± 0%    ~     (all samples are equal)
_FreelistRelease1000K-8     8.05MB ± 0%    8.05MB ± 0%    ~     (all samples are equal)
_FreelistRelease10000K-8    80.4MB ± 0%    80.4MB ± 0%    ~             (p=1.000 n=5+5)
DBBatchAutomatic-8           384kB ± 0%     384kB ± 0%    ~             (p=0.095 n=5+5)
DBBatchSingle-8             17.2MB ± 1%    17.2MB ± 1%    ~             (p=0.310 n=5+5)
DBBatchManual10x100-8        908kB ± 0%     902kB ± 1%    ~             (p=0.730 n=4+5)

name                      old allocs/op  new allocs/op  delta
_FreelistRelease10K-8         5.00 ± 0%      5.00 ± 0%    ~     (all samples are equal)
_FreelistRelease100K-8        5.00 ± 0%      5.00 ± 0%    ~     (all samples are equal)
_FreelistRelease1000K-8       5.00 ± 0%      5.00 ± 0%    ~     (all samples are equal)
_FreelistRelease10000K-8      5.00 ± 0%      5.00 ± 0%    ~     (all samples are equal)
DBBatchAutomatic-8           10.2k ± 0%     10.2k ± 0%  +0.07%          (p=0.032 n=5+5)
DBBatchSingle-8              58.6k ± 0%     59.6k ± 0%  +1.70%          (p=0.008 n=5+5)
DBBatchManual10x100-8        6.02k ± 0%     6.03k ± 0%  +0.17%          (p=0.029 n=4+4)

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